Non-Equilibrium Molecular Dynamics
NASA Astrophysics Data System (ADS)
Ciccotti, Giovanni; Kapral, Raymond; Sergi, Alessandro
Statistical mechanics provides a well-established link between microscopic equilibrium states and thermodynamics. If one considers systems out of equilibrium, the link between microscopic dynamical properties and non-equilibrium macroscopic states is more difficult to establish [1,2]. For systems lying near equilibrium, linear response theory provides a route to derive linear macroscopic laws and the microscopic expressions for the transport properties that enter the constitutive relations. If the system is displaced far from equilibrium, no fully general theory exists to treat such systems. By restricting consideration to a class of non-equilibrium states which arise from perturbations (linear or non-linear) of an equilibrium state, methods can be developed to treat non-equilibrium states. Furthermore, non-equilibrium molecular dynamics (NEMD) simulation methods can be devised to provide estimates for the transport properties of these systems.
Determining equilibrium constants for dimerization reactions from molecular dynamics simulations.
De Jong, Djurre H; Schäfer, Lars V; De Vries, Alex H; Marrink, Siewert J; Berendsen, Herman J C; Grubmüller, Helmut
2011-07-15
With today's available computer power, free energy calculations from equilibrium molecular dynamics simulations "via counting" become feasible for an increasing number of reactions. An example is the dimerization reaction of transmembrane alpha-helices. If an extended simulation of the two helices covers sufficiently many dimerization and dissociation events, their binding free energy is readily derived from the fraction of time during which the two helices are observed in dimeric form. Exactly how the correct value for the free energy is to be calculated, however, is unclear, and indeed several different and contradictory approaches have been used. In particular, results obtained via Boltzmann statistics differ from those determined via the law of mass action. Here, we develop a theory that resolves this discrepancy. We show that for simulation systems containing two molecules, the dimerization free energy is given by a formula of the form ΔG ∝ ln(P(1) /P(0) ). Our theory is also applicable to high concentrations that typically have to be used in molecular dynamics simulations to keep the simulation system small, where the textbook dilute approximations fail. It also covers simulations with an arbitrary number of monomers and dimers and provides rigorous error estimates. Comparison with test simulations of a simple Lennard Jones system with various particle numbers as well as with reference free energy values obtained from radial distribution functions show full agreement for both binding free energies and dimerization statistics.
Bresme, F.; Armstrong, J.
2014-01-07
We report non-equilibrium molecular dynamics simulations of heat transport in models of molecular fluids. We show that the “local” thermal conductivities obtained from non-equilibrium molecular dynamics simulations agree within numerical accuracy with equilibrium Green-Kubo computations. Our results support the local equilibrium hypothesis for transport properties. We show how to use the local dependence of the thermal gradients to quantify the thermal conductivity of molecular fluids for a wide range of thermodynamic states using a single simulation.
Techniques for achieving thermal equilibrium in molecular dynamics calculations for solids
NASA Astrophysics Data System (ADS)
Wu, Ernest Yue; Friauf, Robert J.
1990-06-01
We develop techniques for achieving thermal equilibrium in molecular dynamics calculations for solids. Atoms in a Lennard-Jones solid are initially given random velocities and displacements from their equilibrium positions with suitably scaled Maxwellian distributions. A quantitative criterion for thermal equilibrium of the solid is established by using the equipartition of energy theorem. At high temperatures, thermal expansion is studied, and we introduce a method for adjusting the lattice parameter to ensure zero external pressure. The results of molecular dynamics simulations show agreement with experimental data for rare gas and ionic crystals.
Classical molecular dynamics simulations for non-equilibrium correlated plasmas
NASA Astrophysics Data System (ADS)
Ferri, S.; Calisti, A.; Talin, B.
2017-03-01
A classical molecular dynamics model was recently extended to simulate neutral multi-component plasmas where various charge states of the same atom and electrons coexist. It is used to investigate the plasma effects on the ion charge and on the ionization potential in dense plasmas. Different simulated statistical properties will show that the concept of isolated particles is lost in such correlated plasmas. The charge equilibration is discussed for a carbon plasma at solid density and investigation on the charge distribution and on the ionization potential depression (IPD) for aluminum plasmas is discussed with reference to existing experiments.
Chen, Yunjie; Roux, Benoît
2015-01-14
A family of hybrid simulation methods that combines the advantages of Monte Carlo (MC) with the strengths of classical molecular dynamics (MD) consists in carrying out short non-equilibrium MD (neMD) trajectories to generate new configurations that are subsequently accepted or rejected via an MC process. In the simplest case where a deterministic dynamic propagator is used to generate the neMD trajectories, the familiar Metropolis acceptance criterion based on the change in the total energy ΔE, min[1, exp{-βΔE}], guarantees that the hybrid algorithm will yield the equilibrium Boltzmann distribution. However, the functional form of the acceptance probability is more complex when the non-equilibrium switching process is generated via a non-deterministic stochastic dissipative propagator coupled to a heat bath. Here, we clarify the conditions under which the Metropolis criterion remains valid to rigorously yield a proper equilibrium Boltzmann distribution within hybrid neMD-MC algorithm.
Maćkowiak, Sz; Heyes, D M; Dini, D; Brańka, A C
2016-10-28
The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential
NASA Astrophysics Data System (ADS)
Maćkowiak, Sz.; Heyes, D. M.; Dini, D.; Brańka, A. C.
2016-10-01
The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (˜0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential
NASA Astrophysics Data System (ADS)
Reilly, Anthony M.; Habershon, Scott; Morrison, Carole A.; Rankin, David W. H.
2010-03-01
Path-integral molecular dynamics (PIMD) simulations with an empirical interaction potential have been used to determine the experimental equilibrium structure of solid nitromethane at 4.2 and 15 K. By comparing the time-averaged molecular structure determined in a PIMD simulation to the calculated minimum-energy (zero-temperature) molecular structure, we have derived structural corrections that describe the effects of thermal motion. These corrections were subsequently used to determine the equilibrium structure of nitromethane from the experimental time-averaged structure. We find that the corrections to the intramolecular and intermolecular bond distances, as well as to the torsion angles, are quite significant, particularly for those atoms participating in the anharmonic motion of the methyl group. Our results demonstrate that simple harmonic models of thermal motion may not be sufficiently accurate, even at low temperatures, while molecular simulations employing more realistic potential-energy surfaces can provide important insight into the role and magnitude of anharmonic atomic motions.
A method of solid-solid phase equilibrium calculation by molecular dynamics
NASA Astrophysics Data System (ADS)
Karavaev, A. V.; Dremov, V. V.
2016-12-01
A method for evaluation of solid-solid phase equilibrium curves in molecular dynamics simulation for a given model of interatomic interaction is proposed. The method allows to calculate entropies of crystal phases and provides an accuracy comparable with that of the thermodynamic integration method by Frenkel and Ladd while it is much simpler in realization and less intense computationally. The accuracy of the proposed method was demonstrated in MD calculations of entropies for EAM potential for iron and for MEAM potential for beryllium. The bcc-hcp equilibrium curves for iron calculated for the EAM potential by the thermodynamic integration method and by the proposed one agree quite well.
Xu, J; Kjelstrup, S; Bedeaux, D; Røsjorde, A; Rekvig, L
2006-07-01
Non-equilibrium molecular dynamic (NEMD) simulations have been used to study heat and mass transfer across a vapor-liquid interface for a one-component system using a Lennard-Jones spline potential. It was confirmed that the relation between the surface tension and the surface temperature in the non-equilibrium system was the same as in equilibrium (local equilibrium). Interfacial transfer coefficients were evaluated for the surface, which expressed the heat and mass fluxes in temperature and chemical potential differences across the interfacial region (film). In this analysis it was assumed that the Onsager reciprocal relations were valid. In this paper we extend the number of simulations such that we can calculate all four interface film transfer coefficients along the whole liquid-vapor coexistence curve. We do this analysis both for the case where we use the measurable heat flux on the vapor side and for the case where we use the measurable heat flux on the liquid side. The most important result we found is that the coupling coefficients within the accuracy of the calculation are equal. This is the first verification of the validity of the Onsager relations for transport through a surface using molecular dynamics. The interfacial film transfer coefficients are found to be a function of the surface temperature alone. New expressions are given for the kinetic theory values of these coefficients which only depend on the surface temperature. The NEMD values were found to be in good agreement with these expressions.
NASA Astrophysics Data System (ADS)
Merabia, Samy; Termentzidis, Konstantinos
2012-09-01
In this article, we compare the results of nonequilibrium (NEMD) and equilibrium (EMD) molecular dynamics methods to compute the thermal conductance at the interface between solids. We propose to probe the thermal conductance using equilibrium simulations measuring the decay of the thermally induced energy fluctuations of each solid. We also show that NEMD and EMD give generally speaking inconsistent results for the thermal conductance: Green-Kubo simulations probe the Landauer conductance between two solids which assumes phonons on both sides of the interface to be at equilibrium. On the other hand, we show that NEMD give access to the out-of-equilibrium interfacial conductance consistent with the interfacial flux describing phonon transport in each solid. The difference may be large and reaches typically a factor 5 for interfaces between usual semiconductors. We analyze finite size effects for the two determinations of the interfacial thermal conductance, and show that the equilibrium simulations suffer from severe size effects as compared to NEMD. We also compare the predictions of the two above-mentioned methods—EMD and NEMD—regarding the interfacial conductance of a series of mass mismatched Lennard-Jones solids. We show that the Kapitza conductance obtained with EMD can be well described using the classical diffuse mismatch model (DMM). On the other hand, NEMD simulation results are consistent with an out-of-equilibrium generalization of the acoustic mismatch model (AMM). These considerations are important in rationalizing previous results obtained using molecular dynamics, and help in pinpointing the physical scattering mechanisms taking place at atomically perfect interfaces between solids, which is a prerequisite to understand interfacial heat transfer across real interfaces.
Liu, Xin; Schnell, Sondre K; Simon, Jean-Marc; Bedeaux, Dick; Kjelstrup, Signe; Bardow, André; Vlugt, Thijs J H
2011-11-10
A methodology for computing Fick diffusivities directly from equilibrium molecular dynamics (MD) simulations is presented and validated for acetone-methanol and acetone-tetrachloromethane liquid mixtures. Fick diffusivities are obtained from Maxwell-Stefan (MS) diffusivities and the so-called thermodynamic factor. MS diffusivities describe the friction between different components, while the thermodynamic factor is the concentration derivative of the activity describing the deviation from ideal mixing behavior. It is important to note that all mutual diffusion experiments measure Fick diffusion coefficients, while molecular simulation provides MS diffusivities. The required thermodynamic factor to convert MS into Fick diffusivities and vice versa, however, is usually difficult to extract from both simulations and experiments leaving a gap between theory and application. Here, we employ our novel method to compute the thermodynamic factor from small-scale density fluctuations in equilibrium MD simulations [Chem. Phys. Lett.2011, 504, 199-201]. Previously, this method was developed and validated for molecules with single interaction sites only. In this work, we applied this method to acetone-methanol and acetone-tetrachloromethane liquid mixtures and show that the method also works well in these more complex systems. This provides the missing step to extract Fick diffusion coefficients directly from equilibrium MD simulations. The computed Fick diffusivities of acetone-methanol and acetone-tetrachloromethane mixtures are in excellent agreement with experimental values. The suggested framework thus provides an efficient route to model diffusion in liquids on the basis of a consistent molecular picture.
NASA Astrophysics Data System (ADS)
Cartoixà, Xavier; Dettori, Riccardo; Melis, Claudio; Colombo, Luciano; Rurali, Riccardo
2016-07-01
We study thermal transport in porous Si nanowires (SiNWs) by means of approach-to-equilibrium molecular dynamics simulations. We show that the presence of pores greatly reduces the thermal conductivity, κ, of the SiNWs as long mean free path phonons are suppressed. We address explicitly the dependence of κ on different features of the pore topology—such as the porosity and the pore diameter—and on the nanowire (NW) geometry—diameter and length. We use the results of the molecular dynamics calculations to tune an effective model, which is capable of capturing the dependence of κ on porosity and NW diameter. The model illustrates the failure of Matthiessen's rule to describe the coupling between boundary and pore scattering, which we account for by the inclusion of an additional empirical term.
2006-09-01
Therefore, dynamic quantities of reaction mixtures such as the velocity autocorrelation functions and the diffusion coefficients can be accurately...using the virial expression [25]. A standard NVT molecular dynamics method was em- ployed with the equations of motion solved using the Verlet leapfrog...configurational energy, pressure, and species concen- trations) are compared to quantities calculated by the RxMC approach. Second , the dynamic quantities
NASA Astrophysics Data System (ADS)
English, Niall J.; Clarke, Elaine T.
2013-09-01
Equilibrium and non-equilibrium molecular dynamics (MD) simulations have been performed to investigate thermal-driven break-up of planar CO2 hydrate interfaces in liquid water at 300-320 K. Different guest compositions, at 85%, 95%, and 100% of maximum theoretical occupation, led to statistically-significant differences in the observed initial dissociation rates. The melting temperatures of each interface were estimated, and dissociation rates were observed to be strongly dependent on temperature, with higher dissociation rates at larger over-temperatures vis-à-vis melting. A simple coupled mass and heat transfer model developed previously was applied to fit the observed dissociation profiles, and this helps to identify clearly two distinct régimes of break-up; a second well-defined region is essentially independent of composition and temperature, in which the remaining nanoscale, de facto two-dimensional system's lattice framework is intrinsically unstable. From equilibrium MD of the two-phase systems at their melting point, the relaxation times of the auto-correlation functions of fluctuations in number of enclathrated guest molecules were used as a basis for comparison of the variation in the underlying, non-equilibrium, thermal-driven dissociation rates via Onsager's hypothesis, and statistically significant differences were found, confirming the value of a fluctuation-dissipation approach in this case.
Slip length of water on graphene: limitations of non-equilibrium molecular dynamics simulations.
Kannam, Sridhar Kumar; Todd, B D; Hansen, J S; Daivis, Peter J
2012-01-14
Data for the flow rate of water in carbon nanopores is widely scattered, both in experiments and simulations. In this work, we aim at precisely quantifying the characteristic large slip length and flow rate of water flowing in a planar graphene nanochannel. First, we quantify the slip length using the intrinsic interfacial friction coefficient between water and graphene, which is found from equilibrium molecular dynamics (EMD) simulations. We then calculate the flow rate and the slip length from the streaming velocity profiles obtained using non-equilibrium molecular dynamics (NEMD) simulations and compare with the predictions from the EMD simulations. The slip length calculated from NEMD simulations is found to be extremely sensitive to the curvature of the velocity profile and it possesses large statistical errors. We therefore pose the question: Can a micrometer range slip length be reliably determined using velocity profiles obtained from NEMD simulations? Our answer is "not practical, if not impossible" based on the analysis given as the results. In the case of high slip systems such as water in carbon nanochannels, the EMD method results are more reliable, accurate, and computationally more efficient compared to the direct NEMD method for predicting the nanofluidic flow rate and hydrodynamic boundary condition.
Chavoshi, Saeed Zare; Xu, Shuozhi; Goel, Saurav
2017-06-01
We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid-liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.
NASA Astrophysics Data System (ADS)
Chavoshi, Saeed Zare; Xu, Shuozhi; Goel, Saurav
2017-06-01
We performed molecular dynamics simulations to study the equilibrium melting point of silicon using (i) the solid-liquid coexistence method and (ii) the Gibbs free energy technique, and compared our novel results with the previously published results obtained from the Monte Carlo (MC) void-nucleated melting method based on the Tersoff-ARK interatomic potential (Agrawal et al. Phys. Rev. B 72, 125206. (doi:10.1103/PhysRevB.72.125206)). Considerable discrepancy was observed (approx. 20%) between the former two methods and the MC void-nucleated melting result, leading us to question the applicability of the empirical MC void-nucleated melting method to study a wide range of atomic and molecular systems. A wider impact of the study is that it highlights the bottleneck of the Tersoff-ARK potential in correctly estimating the melting point of silicon.
Equilibrium fractionation of H and O isotopes in water from path integral molecular dynamics
NASA Astrophysics Data System (ADS)
Pinilla, Carlos; Blanchard, Marc; Balan, Etienne; Ferlat, Guillaume; Vuilleumier, Rodolphe; Mauri, Francesco
2014-06-01
The equilibrium fractionation factor between two phases is of importance for the understanding of many planetary and environmental processes. Although thermodynamic equilibrium can be achieved between minerals at high temperature, many natural processes involve reactions between liquids or aqueous solutions and solids. For crystals, the fractionation factor α can be theoretically determined using a statistical thermodynamic approach based on the vibrational properties of the phases. These calculations are mostly performed in the harmonic approximation, using empirical or ab-initio force fields. In the case of aperiodic and dynamic systems such as liquids or solutions, similar calculations can be done using finite-size molecular clusters or snapshots obtained from molecular dynamics (MD) runs. It is however difficult to assess the effect of these approximate models on the isotopic fractionation properties. In this work we present a systematic study of the calculation of the D/H and 18O/16O equilibrium fractionation factors in water for the liquid/vapour and ice/vapour phases using several levels of theory within the simulations. Namely, we use a thermodynamic integration approach based on Path Integral MD calculations (PIMD) and an empirical potential model of water. Compared with standard MD, PIMD takes into account quantum effects in the thermodynamic modeling of systems and the exact fractionation factor for a given potential can be obtained. We compare these exact results with those of modeling strategies usually used, which involve the mapping of the quantum system on its harmonic counterpart. The results show the importance of including configurational disorder for the estimation of isotope fractionation in liquid phases. In addition, the convergence of the fractionation factor as a function of parameters such as the size of the simulated system and multiple isotope substitution is analyzed, showing that isotope fractionation is essentially a local effect in
On determining continuum quantities of non-equilibrium processes via molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Fu, Yao
In this dissertation, a high-fidelity atomistic-to-continuum link for highly non-equilibrium processes has been established by making several modifications to Hardy's theory. Although Hardy's thermomechanical quantities were derived analytically to conserve mass, momentum and energy, they have not been rigorously tested and validated numerically in the past. Hence the first task was to investigate the effectiveness of ensemble averaging in removing thermal fluctuations and compare with conventional time averaging for fcc crystals simulated using both equilibrium and non-equilibrium molecular dynamics (MD) simulations, where the non-equilibrium process was introduced by a shock impact. It has been found that the ensemble averaging has better convergence than time averaging due to the statistical independence of the thermomechanical quantities computed using ensemble averaging. The second task was to test the validity of Hardy's theory by checking if it is able to conserve mass, momentum and energy numerically. A few highly non-equilibrium processes were simulated using MD, including Gaussian wave and shock impact propagation in 1D and 3D fcc crystals. Based on the test results, a new normalization rule has been proposed so that the computed thermomechanical quantities can conserve the fundamental properties more accurately. To a large extent, Hardy's theory has been found to be valid regardless of the width of the localization function, the interatomic potential and crystal structure, and with and without ensemble averaging. To further test the validity of Hardy's theory for more complex non-equilibrium processes, where plastic deformation is accomplished through dislocation glide and slip band emission, a crack propagation problem in iron crystal with a pre-created center crack is simulated using MD. The computed Hardy's thermomechanical quantities can generally conserve mass, momentum and energy. Exceptions have been found around the crack region, where the
Chen, Yunjie; Roux, Benoît
2014-09-21
Hybrid schemes combining the strength of molecular dynamics (MD) and Metropolis Monte Carlo (MC) offer a promising avenue to improve the sampling efficiency of computer simulations of complex systems. A number of recently proposed hybrid methods consider new configurations generated by driving the system via a non-equilibrium MD (neMD) trajectory, which are subsequently treated as putative candidates for Metropolis MC acceptance or rejection. To obey microscopic detailed balance, it is necessary to alter the momentum of the system at the beginning and/or the end of the neMD trajectory. This strict rule then guarantees that the random walk in configurational space generated by such hybrid neMD-MC algorithm will yield the proper equilibrium Boltzmann distribution. While a number of different constructs are possible, the most commonly used prescription has been to simply reverse the momenta of all the particles at the end of the neMD trajectory ("one-end momentum reversal"). Surprisingly, it is shown here that the choice of momentum reversal prescription can have a considerable effect on the rate of convergence of the hybrid neMD-MC algorithm, with the simple one-end momentum reversal encountering particularly acute problems. In these neMD-MC simulations, different regions of configurational space end up being essentially isolated from one another due to a very small transition rate between regions. In the worst-case scenario, it is almost as if the configurational space does not constitute a single communicating class that can be sampled efficiently by the algorithm, and extremely long neMD-MC simulations are needed to obtain proper equilibrium probability distributions. To address this issue, a novel momentum reversal prescription, symmetrized with respect to both the beginning and the end of the neMD trajectory ("symmetric two-ends momentum reversal"), is introduced. Illustrative simulations demonstrate that the hybrid neMD-MC algorithm robustly yields a correct
Non-equilibrium molecular dynamics simulations of spall in single crystal tantalum
NASA Astrophysics Data System (ADS)
Hahn, Eric N.; Germann, Timothy C.; Ravelo, Ramon J.; Hammerberg, James E.; Meyers, Marc A.
2017-01-01
Ductile tensile failure of tantalum is examined through large scale non-equilibrium molecular dynamics simulations. Several loading schemes including flyer plate impact, decaying shock loading via a frozen piston, and quasi-isentropic (constant strain-rate) expansion are employed to span tensile strain-rates of 108 to 1014 per second. Single crystals of <001> orientation are specifically evaluated to eliminate grain boundary effects. Heterogeneous void nucleation occurs principally at the intersection of deformation twins in single crystals. At high strain rates, multiple spall events occur throughout the material and voids continue to nucleate until relaxation waves arrive from adjacent events. At ultra-high strain rates, those approaching or exceeding the atomic vibrational frequency, spall strength saturates near the maximum theoretical spall strength.
Liang, Zhi; Evans, William; Keblinski, Pawel
2013-02-01
The thermal conductance at solid-gas interfaces with different interfacial bonding strengths is calculated through Green-Kubo equilibrium molecular dynamics (EMD) simulations. Due to the finite size of the simulation system, the long-time integral of the time correlation function of heat power across the solid-gas interface exhibits an exponential decay, which contains the information on interfacial thermal conductance. If an adsorbed gas layer is formed on the solid surface, it is found that the solid-gas interface needs to be defined at a plane outside the adsorbed layer so as to obtain the correct result from the Green-Kubo formula. The EMD simulation result agrees very well with that obtained from nonequilibrium molecular dynamics simulations. By calculating the average solid-gas interaction time as a function of solid-gas interaction strength, we find the incident gas atoms thermalize with the metal surface much more rapidly when the surface is covered by adsorbed gas molecules.
Temperature control in molecular dynamic simulations of non-equilibrium processes
NASA Astrophysics Data System (ADS)
Toton, Dawid; Lorenz, Christian D.; Rompotis, Nikolaos; Martsinovich, Natalia; Kantorovich, Lev
2010-02-01
Thermostats are often used in various condensed matter problems, e.g. when a biological molecule undergoes a transformation in a solution, a crystal surface is irradiated with energetic particles, a crack propagates in a solid upon applied stress, two surfaces slide with respect to each other, an excited local phonon dissipates its energy into a crystal bulk, and so on. In all of these problems, as well as in many others, there is an energy transfer between different local parts of the entire system kept at a constant temperature. Very often, when modelling such processes using molecular dynamics simulations, thermostatting is done using strictly equilibrium approaches serving to describe the NVT ensemble. In this paper we critically discuss the applicability of such approaches to non-equilibrium problems, including those mentioned above, and stress that the correct temperature control can only be achieved if the method is based on the generalized Langevin equation (GLE). Specifically, we emphasize that a meaningful compromise between computational efficiency and a physically appropriate implementation of the NVT thermostat can be achieved, at least for solid state and surface problems, if the so-called stochastic boundary conditions (SBC), recently derived from the GLE (Kantorovich and Rompotis 2008 Phys. Rev. B 78 094305), are used. For SBC, the Langevin thermostat is only applied to the outer part of the simulated fragment of the entire system which borders the surrounding environment (not considered explicitly) serving as a heat bath. This point is illustrated by comparing the performance of the SBC and some of the equilibrium thermostats in two problems: (i) irradiation of the Si(001) surface with an energetic CaF2 molecule using an ab initio density functional theory based method, and (ii) the tribology of two amorphous SiO2 surfaces coated with self-assembled monolayers of methyl-terminated hydrocarbon alkoxylsilane molecules using a classical atomistic
Non-equilibrium dynamics in disordered materials: Ab initio molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Ohmura, Satoshi; Nagaya, Kiyonobu; Shimojo, Fuyuki; Yao, Makoto
2015-08-01
The dynamic properties of liquid B2O3 under pressure and highly-charged bromophenol molecule are studied by using molecular dynamics (MD) simulations based on density functional theory (DFT). Diffusion properties of covalent liquids under high pressure are very interesting in the sense that they show unexpected pressure dependence. It is found from our simulation that the magnitude relation of diffusion coefficients for boron and oxygen in liquid B2O3 shows the anomalous pressure dependence. The simulation clarified the microscopic origin of the anomalous diffusion properties. Our simulation also reveals the dissociation mechanism in the coulomb explosion of the highly-charged bromophenol molecule. When the charge state n is 6, hydrogen atom in the hydroxyl group dissociates at times shorter than 20 fs while all hydrogen atoms dissociate when n is 8. After the hydrogen dissociation, the carbon ring breaks at about 100 fs. There is also a difference on the mechanism of the ring breaking depending on charge states, in which the ring breaks with expanding (n = 6) or shrink (n = 8).
Non-equilibrium dynamics in disordered materials: Ab initio molecular dynamics simulations
Ohmura, Satoshi; Nagaya, Kiyonobu; Yao, Makoto; Shimojo, Fuyuki
2015-08-17
The dynamic properties of liquid B{sub 2}O{sub 3} under pressure and highly-charged bromophenol molecule are studied by using molecular dynamics (MD) simulations based on density functional theory (DFT). Diffusion properties of covalent liquids under high pressure are very interesting in the sense that they show unexpected pressure dependence. It is found from our simulation that the magnitude relation of diffusion coefficients for boron and oxygen in liquid B{sub 2}O{sub 3} shows the anomalous pressure dependence. The simulation clarified the microscopic origin of the anomalous diffusion properties. Our simulation also reveals the dissociation mechanism in the coulomb explosion of the highly-charged bromophenol molecule. When the charge state n is 6, hydrogen atom in the hydroxyl group dissociates at times shorter than 20 fs while all hydrogen atoms dissociate when n is 8. After the hydrogen dissociation, the carbon ring breaks at about 100 fs. There is also a difference on the mechanism of the ring breaking depending on charge states, in which the ring breaks with expanding (n = 6) or shrink (n = 8)
NASA Astrophysics Data System (ADS)
Wang, Zuyuan; Ruan, Xiulin
2017-01-01
Equilibrium molecular dynamics (EMD) simulations with the Green-Kubo formula and nonequilibrium molecular dynamics (NEMD) simulations with the Fourier's Law are two widely used methods for calculating thermal conductivities of materials. It is well known that both methods suffer from domain size effects, especially for NEMD. But the underlying mechanisms and their comparison have not been much quantitatively studied before. In this paper, we investigate their domain size effects by using crystalline silicon at 1000 K, graphene at 300 K, and silicene at 300 K as model material systems. The thermal conductivity of silicon from EMD simulations increases normally with the increasing domain size and converges at a size of around 4 ×4 ×4 nm3 . The converging trend agrees well with the wavelength-accumulated thermal conductivity. The thermal conductivities of graphene and silicene from EMD simulations decrease abnormally with the increasing domain size and converge at a size of around 10 ×10 nm2 . We ascribe the anomalous size effect to the fact that as the domain size increases, the effect of more phonon scattering processes (particularly the flexural phonons) dominates over the effect of more phonon modes contributing to the thermal conductivity. The thermal conductivities of the three material systems from NEMD simulations all show normal domain size effects, although their dependences on the domain size differ. The converging trends agree with the mean free path accumulation of thermal conductivity. This study provides new insights that other than some exceptions, the domain size effects of EMD and NEMD are generally associated with wavelength and mean free path accumulations of thermal conductivity, respectively. Since phonon wavelength spans over a much narrower range than mean free path, EMD usually has less significant domain size effect than NEMD.
NASA Astrophysics Data System (ADS)
Gheribi, Aïmen E.; Salanne, Mathieu; Chartrand, Patrice
2015-03-01
The composition dependence of thermal transport properties of the (Na,K)Cl rocksalt solid solution is investigated through equilibrium molecular dynamics (EMD) simulations in the entire range of composition and the results are compared with experiments published in recent work [Gheribi et al., J. Chem. phys. 141, 104508 (2014)]. The thermal diffusivity of the (Na,K)Cl solid solution has been measured from 473 K to 823 K using the laser flash technique, and the thermal conductivity was deduced from critically assessed data of heat capacity and density. The thermal conductivity was also predicted at 900 K in the entire range of composition by a series of EMD simulations in both NPT and NVT statistical ensembles using the Green-Kubo theory. The aim of the present paper is to provide an objective analysis of the capability of EMD simulations in predicting the composition dependence of the thermal transport properties of halide solid solutions. According to the Klemens-Callaway [P. G. Klemens, Phys. Rev. 119, 507 (1960) and J. Callaway and H. C. von Bayer, Phys. Rev. 120, 1149 (1960)] theory, the thermal conductivity degradation of the solid solution is explained by mass and strain field fluctuations upon the phonon scattering cross section. A rigorous analysis of the consistency between the theoretical approach and the EMD simulations is discussed in detail.
Simulating ionic thermal trasport by equilibrium ab-initio molecular dynamics
NASA Astrophysics Data System (ADS)
Marcolongo, Aris; Umari, Paolo; Baroni, Stefano
2014-03-01
The Green-Kubo approach to thermal transport is often considered to be incompatible with ab-initio molecular dynamics (AIMD) because a suitable quantum-mechanical definition of the heat current is not readily available, due to the ill-definedness of the microscopic energy density to which it is related by the continuity equation. We argue that a similar difficulty actually exists in classical mechanics as well, and we address the conditions that have to be fulfilled in order for the physically well defined transport coefficients to be independent of the ill defined microscopic energy density from which they derive. We then provide two alternative approaches to calculating thermal conductivites from equilibrium AIMD. The first is based on the Green-Kubo formula, supplemented with an expression for the energy current, which is a generalization of Thouless' expression for the adiabatic charge current. The second approach, which avoids the recourse to an energy current altogether, rests on an efficient and accurate extrapolation to infinite wavelengths of the energy-density time correlation functions. The two methods are compared on a simple classical test bed, and their implementation in AIMD is demonstrated with the calculation of the thermal conductivity of simple fluids.
Non-equilibrium molecular dynamics simulation of the unstirred layer in the osmotically driven flow
NASA Astrophysics Data System (ADS)
Konno, Keito; Itano, Tomoaki; Seki, Masako
2015-11-01
We studied the solvent flows driven by the osmotic pressure difference across the semi-permeable membrane. The flow penetrating from the low concentration side transports away solutes adjacent of the membrane, so that the concentration is reduced significantly only at the vicinity of the membrane. It is expected that the relatively low solute concentration develops into a thin boundary layer in the vicinity of the membrane in the case of absence of external stirring process, which is termed as un-stirred layer (USL). To investigate concentration distribution in USL, we carried out non-equilibrium molecular dynamics simulations. The flows driven by th osmotic pressure are idealized as 2 dimensional hard disk model, which is composed of solvent and solute molecules. The membrane is modeled as a medium composed of stationary parallel rods distributed by a spatial interval, which is less than the diameter of the solute molecules. The following results were obtained from the numerical simulation. First, the thickness of USL, which was estimated from the obtained concentration distribution, is on the order of a length determined by mean free path. Second, USL was semicircle the center of which is on the end of pore of membrane.
Gheribi, Aïmen E. Chartrand, Patrice; Salanne, Mathieu
2015-03-28
The composition dependence of thermal transport properties of the (Na,K)Cl rocksalt solid solution is investigated through equilibrium molecular dynamics (EMD) simulations in the entire range of composition and the results are compared with experiments published in recent work [Gheribi et al., J. Chem. phys. 141, 104508 (2014)]. The thermal diffusivity of the (Na,K)Cl solid solution has been measured from 473 K to 823 K using the laser flash technique, and the thermal conductivity was deduced from critically assessed data of heat capacity and density. The thermal conductivity was also predicted at 900 K in the entire range of composition by a series of EMD simulations in both NPT and NVT statistical ensembles using the Green-Kubo theory. The aim of the present paper is to provide an objective analysis of the capability of EMD simulations in predicting the composition dependence of the thermal transport properties of halide solid solutions. According to the Klemens-Callaway [P. G. Klemens, Phys. Rev. 119, 507 (1960) and J. Callaway and H. C. von Bayer, Phys. Rev. 120, 1149 (1960)] theory, the thermal conductivity degradation of the solid solution is explained by mass and strain field fluctuations upon the phonon scattering cross section. A rigorous analysis of the consistency between the theoretical approach and the EMD simulations is discussed in detail.
NASA Technical Reports Server (NTRS)
Haskins, Justin; Kinaci, Alper; Sevik, Cem; Cagin, Tahir
2012-01-01
It is widely known that graphene and many of its derivative nanostructures have exceedingly high reported thermal conductivities (up to 4000 W/mK at 300 K). Such attractive thermal properties beg the use of these structures in practical devices; however, to implement these materials while preserving transport quality, the influence of structure on thermal conductivity should be thoroughly understood. For graphene nanostructures, having average phonon mean free paths on the order of one micron, a primary concern is how size influences the potential for heat conduction. To investigate this, we employ a novel technique to evaluate the lattice thermal conductivity from the Green-Kubo relations and equilibrium molecular dynamics in systems where phonon-boundary scattering dominates heat flow. Specifically, the thermal conductivities of graphene nanoribbons and carbon nanotubes are calculated in sizes up to 3 microns, and the relative influence of boundary scattering on thermal transport is determined to be dominant at sizes less than 1 micron, after which the thermal transport largely depends on the quality of the nanostructure interface. The method is also extended to carbon nanostructures (fullerenes) where phonon confinement, as opposed to boundary scattering, dominates, and general trends related to the influence of curvature on thermal transport in these materials are discussed.
NASA Astrophysics Data System (ADS)
Furuta, Yuma; Surblys, Donatas; Yamaguchi, Yastaka
2016-11-01
Molecular dynamics simulations of the equilibrium wetting behavior of hemi-cylindrical argon droplets on solid surfaces with a periodic roughness were carried out. The rough solid surface is located at the bottom of the calculation cell with periodic boundary conditions in surface lateral directions and mirror boundary condition at the top boundary. Similar to on a smooth surface, the change of the cosine of the droplet contact angle was linearly correlated to the potential well depth of the inter-atomic interaction between liquid and solid on a surface with a short roughness period while the correlation was deviated on one with a long roughness period. To further investigate this feature, solid-liquid, solid-vapor interfacial free energies per unit projected area of solid surface were evaluated by using the thermodynamic integration method in independent quasi-one-dimensional simulation systems with a liquid-solid interface or vapor-solid interface on various rough solid surfaces at a constant pressure. The cosine of the apparent contact angles estimated from the density profile of the droplet systems corresponded well with ones calculated from Young's equation using the interfacial energies evaluated in the quasi-one dimensional systems.
NASA Astrophysics Data System (ADS)
English, Niall J.; Garate, José-A.
2016-08-01
An extensive suite of non-equilibrium molecular-dynamics simulation has been performed for ˜0.85-0.9 μs of human aquaporin 4 in the absence and presence of externally applied static and alternating electric fields applied along the channels (in both axial directions in the static case, taken as the laboratory z-axis). These external fields were of 0.0065 V/Å (r.m.s.) intensity (of the same order as physiological electrical potentials); alternating fields ranged in frequency from 2.45 to 500 GHz. In-pore gating dynamics was studied, particularly of the relative propensities for "open" and "closed" states of the conserved arginines in the arginine/aromatic area (itself governed in no small part by external-field response of the dipolar alignment of the histidine-201 residue in the selectivity filter). In such a manner, the intimate connection of field-response governing "two-state" histidine states was established statistically and mechanistically. Given the appreciable size of the energy barriers for histidine-201 alignment, we have also performed non-equilibrium metadynamics/local-elevation of static fields applied along both directions to construct the free-energy landscape thereof in terms of external-field direction, elucidating the importance of field direction on energetics. We conclude from direct measurement of deterministic molecular dynamics in conjunction with applied-field metadynamics that the intrinsic electric field within the channel points along the +z-axis, such that externally applied static fields in this direction serve to "open" the channel in the selectivity-filter and the asparagine-proline-alanine region.
English, Niall J; Garate, José-A
2016-08-28
An extensive suite of non-equilibrium molecular-dynamics simulation has been performed for ∼0.85-0.9 μs of human aquaporin 4 in the absence and presence of externally applied static and alternating electric fields applied along the channels (in both axial directions in the static case, taken as the laboratory z-axis). These external fields were of 0.0065 V/Å (r.m.s.) intensity (of the same order as physiological electrical potentials); alternating fields ranged in frequency from 2.45 to 500 GHz. In-pore gating dynamics was studied, particularly of the relative propensities for "open" and "closed" states of the conserved arginines in the arginine/aromatic area (itself governed in no small part by external-field response of the dipolar alignment of the histidine-201 residue in the selectivity filter). In such a manner, the intimate connection of field-response governing "two-state" histidine states was established statistically and mechanistically. Given the appreciable size of the energy barriers for histidine-201 alignment, we have also performed non-equilibrium metadynamics/local-elevation of static fields applied along both directions to construct the free-energy landscape thereof in terms of external-field direction, elucidating the importance of field direction on energetics. We conclude from direct measurement of deterministic molecular dynamics in conjunction with applied-field metadynamics that the intrinsic electric field within the channel points along the +z-axis, such that externally applied static fields in this direction serve to "open" the channel in the selectivity-filter and the asparagine-proline-alanine region.
Non-equilibrium conformational dynamics in the function of molecular chaperones.
Barducci, Alessandro; De Los Rios, Paolo
2015-02-01
Why do chaperones need ATP hydrolysis to help proteins reach their native, functional states? In this review, we highlight the most recent experimental and theoretical evidences suggesting that ATP hydrolysis allows molecular chaperones to escape the bounds imposed by equilibrium thermodynamics. We argue here that energy consumption must be fully taken into account to understand the mechanism of these intrinsically non-equilibrium machines and we propose a novel perspective in the way the relation between function and ATP hydrolysis is viewed. Copyright © 2015 Elsevier Ltd. All rights reserved.
Lemarchand, Claire A; Bailey, Nicholas P; Todd, Billy D; Daivis, Peter J; Hansen, Jesper S
2015-06-28
The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related not only to the inter- and intramolecular alignments of the solvent molecules but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole sample due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions, and associating polymer networks is discussed.
NASA Astrophysics Data System (ADS)
Lemarchand, Claire A.; Bailey, Nicholas P.; Todd, Billy D.; Daivis, Peter J.; Hansen, Jesper S.
2015-06-01
The rheology and molecular structure of a model bitumen (Cooee bitumen) under shear are investigated in the non-Newtonian regime using non-equilibrium molecular dynamics simulations. The shear viscosity, normal stress differences, and pressure of the bitumen mixture are computed at different shear rates and different temperatures. The model bitumen is shown to be a shear-thinning fluid at all temperatures. In addition, the Cooee model is able to reproduce experimental results showing the formation of nanoaggregates composed of stacks of flat aromatic molecules in bitumen. These nanoaggregates are immersed in a solvent of saturated hydrocarbon molecules. At a fixed temperature, the shear-shinning behavior is related not only to the inter- and intramolecular alignments of the solvent molecules but also to the decrease of the average size of the nanoaggregates at high shear rates. The variation of the viscosity with temperature at different shear rates is also related to the size and relative composition of the nanoaggregates. The slight anisotropy of the whole sample due to the nanoaggregates is considered and quantified. Finally, the position of bitumen mixtures in the broad literature of complex systems such as colloidal suspensions, polymer solutions, and associating polymer networks is discussed.
Zhang, Yong; Otani, Akihito; Maginn, Edward J
2015-08-11
Equilibrium molecular dynamics is often used in conjunction with a Green-Kubo integral of the pressure tensor autocorrelation function to compute the shear viscosity of fluids. This approach is computationally expensive and is subject to a large amount of variability because the plateau region of the Green-Kubo integral is difficult to identify unambiguously. Here, we propose a time decomposition approach for computing the shear viscosity using the Green-Kubo formalism. Instead of one long trajectory, multiple independent trajectories are run and the Green-Kubo relation is applied to each trajectory. The averaged running integral as a function of time is fit to a double-exponential function with a weighting function derived from the standard deviation of the running integrals. Such a weighting function minimizes the uncertainty of the estimated shear viscosity and provides an objective means of estimating the viscosity. While the formal Green-Kubo integral requires an integration to infinite time, we suggest an integration cutoff time tcut, which can be determined by the relative values of the running integral and the corresponding standard deviation. This approach for computing the shear viscosity can be easily automated and used in computational screening studies where human judgment and intervention in the data analysis are impractical. The method has been applied to the calculation of the shear viscosity of a relatively low-viscosity liquid, ethanol, and relatively high-viscosity ionic liquid, 1-n-butyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([BMIM][Tf2N]), over a range of temperatures. These test cases show that the method is robust and yields reproducible and reliable shear viscosity values.
NASA Astrophysics Data System (ADS)
Vermorel, Romain; Oulebsir, Fouad; Galliero, Guillaume
2017-09-01
The computation of diffusion coefficients in molecular systems ranks among the most useful applications of equilibrium molecular dynamics simulations. However, when dealing with the problem of fluid diffusion through vanishingly thin interfaces, classical techniques are not applicable. This is because the volume of space in which molecules diffuse is ill-defined. In such conditions, non-equilibrium techniques allow for the computation of transport coefficients per unit interface width, but their weak point lies in their inability to isolate the contribution of the different physical mechanisms prone to impact the flux of permeating molecules. In this work, we propose a simple and accurate method to compute the diffusional transport coefficient of a pure fluid through a planar interface from equilibrium molecular dynamics simulations, in the form of a diffusion coefficient per unit interface width. In order to demonstrate its validity and accuracy, we apply our method to the case study of a dilute gas diffusing through a smoothly repulsive single-layer porous solid. We believe this complementary technique can benefit to the interpretation of the results obtained on single-layer membranes by means of complex non-equilibrium methods.
Vermorel, Romain; Oulebsir, Fouad; Galliero, Guillaume
2017-09-14
The computation of diffusion coefficients in molecular systems ranks among the most useful applications of equilibrium molecular dynamics simulations. However, when dealing with the problem of fluid diffusion through vanishingly thin interfaces, classical techniques are not applicable. This is because the volume of space in which molecules diffuse is ill-defined. In such conditions, non-equilibrium techniques allow for the computation of transport coefficients per unit interface width, but their weak point lies in their inability to isolate the contribution of the different physical mechanisms prone to impact the flux of permeating molecules. In this work, we propose a simple and accurate method to compute the diffusional transport coefficient of a pure fluid through a planar interface from equilibrium molecular dynamics simulations, in the form of a diffusion coefficient per unit interface width. In order to demonstrate its validity and accuracy, we apply our method to the case study of a dilute gas diffusing through a smoothly repulsive single-layer porous solid. We believe this complementary technique can benefit to the interpretation of the results obtained on single-layer membranes by means of complex non-equilibrium methods.
Małolepsza, Edyta; Secor, Maxim; Keyes, Tom
2015-10-22
A prescription for sampling isobaric generalized ensembles with molecular dynamics is presented and applied to the generalized replica exchange method (gREM), which was designed to simulate first-order phase transitions. The properties of the isobaric gREM ensemble are discussed, and a study is presented for the liquid-vapor equilibrium of the guest molecules given for gas hydrate formation with the mW water model. Phase diagrams, critical parameters, and a law of corresponding states are obtained.
Variable timestep algorithm for molecular dynamics simulation of non-equilibrium processes
NASA Astrophysics Data System (ADS)
Marks, Nigel A.; Robinson, Marc
2015-06-01
A simple, yet robust variable timestep algorithm is developed for use in molecular dynamics simulations of energetic processes. Single-particle Kepler orbits are studied to study the relationship between trajectory properties and the critical timestep for constant integration error. Over a wide variety of conditions the magnitude of the maximum force is found to correlate linearly with the inverse critical timestep. Other quantities used in the literature such as the time derivative of the force and the product of the velocity and force also show reasonable correlations, but not to the same extent. Application of the corresponding metric | |Fmax | | Δt in molecular dynamics simulation of radiation damage in graphite shows that the scheme is both straightforward to implement and effective. In tests on a 1 keV cascade the timestep varies by over two orders of magnitude with minimal loss of energy conservation.
NASA Astrophysics Data System (ADS)
Pinilla, Carlos; Blanchard, Marc; Balan, Etienne; Natarajan, Suresh K.; Vuilleumier, Rodolphe; Mauri, Francesco
2015-08-01
The theoretical determination of the isotopic fractionation between an aqueous solution and a mineral is of utmost importance in Earth sciences. While for crystals, it is well established that equilibrium isotopic fractionation factors can be calculated using a statistical thermodynamic approach based on the vibrational properties, several theoretical methods are currently used to model ions in aqueous solution. In this work, we present a systematic study to determine the reduced partition function ratio (β-factor) of aqueous Mg2+ using several levels of theory within the simulations. In particular, using an empirical force field, we compare and discuss the performance of the exact results obtained from path integral molecular dynamics (PIMD) simulations, with respect to the more traditional methods based on vibrational properties and the cluster approximation. The results show the importance of including configurational disorder for the estimation of the equilibrium isotope fractionation factor. We also show that using the vibrational frequencies computed from snapshots taken from equilibrated classical molecular dynamics represents a good approximation for the study of aqueous ions. Based on these conclusions, the β-factor of aqueous Mg2+ have been estimated from a Car-Parrinello molecular dynamics (CPMD) simulation with an ab initio force field, and combined with the β-factors of carbonate minerals (magnesite, dolomite, calcite and aragonite). Mg β-factor of Mg-bearing aragonite, calculated here for the first time, displays a lower value than the three other carbonate minerals. This is explained by a strong distortion of the cationic site leading to a decrease of the coordination number during Ca-Mg substitution. Overall, the equilibrium magnesium isotope fractionation factors between aqueous Mg2+ and carbonate minerals that derive from this methodological study support the previous theoretical results obtained from embedded cluster models.
NASA Astrophysics Data System (ADS)
Dongre, B.; Wang, T.; Madsen, G. K. H.
2017-07-01
Different molecular dynamics methods like the direct method, the Green-Kubo (GK) method and homogeneous non-equilibrium molecular dynamics (HNEMD) method have been widely used to calculate lattice thermal conductivity ({κ }{\\ell }). While the first two methods have been used and compared quite extensively, there is a lack of comparison of these methods with the HNEMD method. Focusing on the underlying computational parameters, we present a detailed comparison of the GK and HNEMD methods for both bulk and vacancy Si using the Stillinger-Weber potential. For the bulk calculations, we find both methods to perform well and yield {κ }{\\ell } within acceptable uncertainties. In case of the vacancy calculations, HNEMD method has a slight advantage over the GK method as it becomes computationally cheaper for lower {κ }{\\ell } values. This study could promote the application of HNEMD method in {κ }{\\ell } calculations involving other lattice defects like nanovoids, dislocations, interfaces.
NASA Astrophysics Data System (ADS)
Sääskilahti, K.; Oksanen, J.; Tulkki, J.; McGaughey, A. J. H.; Volz, S.
2016-12-01
The frequency-dependent mean free paths (MFPs) of vibrational heat carriers in amorphous silicon are predicted from the length dependence of the spectrally decomposed heat current (SDHC) obtained from non-equilibrium molecular dynamics simulations. The results suggest a (frequency)- 2 scaling of the room-temperature MFPs below 5 THz. The MFPs exhibit a local maximum at a frequency of 8 THz and fall below 1 nm at frequencies greater than 10 THz, indicating localized vibrations. The MFPs extracted from sub-10 nm system-size simulations are used to predict the length-dependence of thermal conductivity up to system sizes of 100 nm and good agreement is found with independent molecular dynamics simulations. Weighting the SDHC by the frequency-dependent quantum occupation function provides a simple and convenient method to account for quantum statistics and provides reasonable agreement with the experimentally-measured trend and magnitude.
Oh, Suk Yung; Bae, Young Chan
2010-07-15
The method presented in this paper was developed to predict liquid-liquid equilibria in ternary liquid mixtures by using a combination of a thermodynamic model and molecular dynamics simulations. In general, common classical thermodynamic models have many parameters which are determined by fitting a model with experimental data. This proposed method, however, provides a simple procedure for calculating liquid-liquid equilibria utilizing binary interaction parameters and molecular size parameters determined from molecular dynamics simulations. This method was applied to mixtures containing water, hydrocarbons, alcohols, chlorides, ketones, acids, and other organic liquids over various temperature ranges. The predicted results agree well with the experimental data without the use of adjustable parameters.
Pařez, Stanislav; Předota, Milan
2012-03-14
We generalize a technique for determination of the shear viscosity of mixtures in planar slabs using non-equilibrium computer simulations by applying an external force parallel to the surface generating Poiseuille flow. The distance-dependent viscosity of the mixture, given as a function of the distance from the surface, is determined by analysis of the resulting velocity profiles of all species. We present results for a highly non-ideal water + methanol mixture in the whole concentration range between rutile (TiO(2)) walls. The bulk results are compared to the existing equilibrium molecular dynamics and experimental data while the inhomogeneous viscosity profiles at the interface are interpreted using the structural data and information on hydrogen bonding.
Malolepsza, Edyta; Secor, Maxim; Keyes, Tom
2015-09-23
A prescription for sampling isobaric generalized ensembles with molecular dynamics is presented and applied to the generalized replica exchange method (gREM), which was designed for simulating first-order phase transitions. The properties of the isobaric gREM ensemble are discussed and a study is presented of the liquid-vapor equilibrium of the guest molecules given for gas hydrate formation with the mW water model. As a result, phase diagrams, critical parameters, and a law of corresponding states are obtained.
NASA Astrophysics Data System (ADS)
Ngo, Son Tung; Nguyen, Minh Tung; Nguyen, Minh Tho
2017-05-01
The absolute binding free energy of an inhibitor to HIV-1 Protease (PR) was determined throughout evaluation of the non-bonded interaction energy difference between the two bound and unbound states of the inhibitor and surrounding molecules by the fast pulling of ligand (FPL) process using non-equilibrium molecular dynamics (NEMD) simulations. The calculated free energy difference terms help clarifying the nature of the binding. Theoretical binding affinities are in good correlation with experimental data, with R = 0.89. The paradigm used is able to rank two inhibitors having the maximum difference of ∼1.5 kcal/mol in absolute binding free energies.
Malolepsza, Edyta; Secor, Maxim; Keyes, Tom
2015-09-23
A prescription for sampling isobaric generalized ensembles with molecular dynamics is presented and applied to the generalized replica exchange method (gREM), which was designed for simulating first-order phase transitions. The properties of the isobaric gREM ensemble are discussed and a study is presented of the liquid-vapor equilibrium of the guest molecules given for gas hydrate formation with the mW water model. As a result, phase diagrams, critical parameters, and a law of corresponding states are obtained.
Nilsen, Ian A.; Osborne, Derek G.; White, Aaron M.; Anna, Jessica M.; Kubarych, Kevin J.
2014-10-07
Using rapidly acquired spectral diffusion, a recently developed variation of heterodyne detected infrared photon echo spectroscopy, we observe ∼3 ps solvent independent spectral diffusion of benzene chromium tricarbonyl (C{sub 6}H{sub 6}Cr(CO){sub 3}, BCT) in a series of nonpolar linear alkane solvents. The spectral dynamics is attributed to low-barrier internal torsional motion. This tripod complex has two stable minima corresponding to staggered and eclipsed conformations, which differ in energy by roughly half of k{sub B}T. The solvent independence is due to the relative size of the rotor compared with the solvent molecules, which create a solvent cage in which torsional motion occurs largely free from solvent damping. Since the one-dimensional transition state is computed to be only 0.03 k{sub B}T above the higher energy eclipsed conformation, this model system offers an unusual, nearly barrierless reaction, which nevertheless is characterized by torsional coordinate dependent vibrational frequencies. Hence, by studying the spectral diffusion of the tripod carbonyls, it is possible to gain insight into the fundamental dynamics of internal rotational motion, and we find some evidence for the importance of non-diffusive ballistic motion even in the room-temperature liquid environment. Using several different approaches to describe equilibrium kinetics, as well as the influence of reactive dynamics on spectroscopic observables, we provide evidence that the low-barrier torsional motion of BCT provides an excellent test case for detailed studies of the links between chemical exchange and linear and nonlinear vibrational spectroscopy.
Molecular equilibrium with condensation
NASA Astrophysics Data System (ADS)
Sharp, C. M.; Huebner, W. F.
1990-02-01
Minimization of the Gibbs energy of formation for species of chemical elements and compounds in their gas and condensed phases determines their relative abundances in a mixture in chemical equilibrium. The procedure is more general and more powerful than previous abundance determinations in multiphase astrophysical mixtures. Some results for astrophysical equations of state are presented, and the effects of condensation on opacity are briefly indicated.
Equilibrium moisture content of a crosslinked epoxy network via molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Stoffels, M. T.; Staiger, M. P.; Bishop, C. M.
2016-06-01
This study presents molecular dynamics (MD) simulation methods for determining the solubility limit of water in a crosslinked epoxy network. Procedures are first presented for dynamically crosslinking an epoxy network consisting of diglycidyl ether bisphenol a (DGEBA) and isophorone diamine (IPD). Water molecules are then introduced into the crosslinked DGEBA-IPD structure. The excess chemical potential for the absorbed water was determined through combining thermodynamic integration and Widom’s test particle insertion methods. The limiting moisture uptake of the epoxy structure was determined through comparing the reduced chemical potential of the water held within the epoxy to that of pure water. The DGEBA-IPD epoxy system was found to have a moisture solubility of 3.50-3.75 wt.% when immersed in water at 300 K.
Equilibrium properties of the reaction H2 ⇌ 2H by classical molecular dynamics simulations.
Skorpa, Ragnhild; Simon, Jean-Marc; Bedeaux, Dick; Kjelstrup, Signe
2014-01-21
We have developed a classical molecular dynamics model for the hydrogen dissociation reaction, containing two- and three-particle potentials derived by Kohen, Tully and Stillinger. Two fluid densities were investigated for a wide range of temperatures, and 11 fluid densities were considered for one temperature. We report the temperature range where the degree of reaction is significant, and also where a stable molecule dominates the population in the energy landscape. The three-particle potential, which is essential for the reaction model and seldom studied, together with the two-particle interaction lead to a large effective excluded volume diameter of the molecules in the molecular fluid. The three-particle interaction was also found to give a large positive contribution to the pressure of the reacting mixture at high density and/or low temperatures. From knowledge of the dissociation constant of the reaction and the fluid pressure, we estimated the standard enthalpy of the dissociation reaction to be 430 kJ mol(-1) (ρ = 0.0695 g cm(-3)) and 380 kJ mol(-1) (ρ = 0.0191 g cm(-3)). These values are in good agreement with the experimental vaule of 436 kJ mol(-1) under ambient pressure. The model is consistent with a Lennard-Jones model of the molecular fluid, and may facilitate studies of the impact of chemical reactions on transport systems.
NASA Astrophysics Data System (ADS)
Rohith, P.; Sainath, G.; Choudhary, B. K.
2017-05-01
In face centered cubic systems, due to decrease in energy all perfect dislocations dissociates into two Shockley partials separated by stacking fault width. The stacking fault width, which influences the deformation behavior depends on many factors such as composition, stacking fault energy, temperature, surface energy and applied stress. Additionally in thin films, thickness also influences the stacking fault width of dissociated dislocations. In this paper, we investigate the effect of thin film thickness on stacking fault width in Cu using molecular dynamics simulations. The results indicate that with increase in film thickness from 1.25 nm to 11 nm, the stacking fault width increases from 1.6 nm to 3.12 nm. A bi-linear behavior has been observed. Above 11 nm thickness, the width of stacking fault has attained a saturation at higher thickness. This thickness dependent dissociation has been explained using the concept of image dislocations and associated image forces.
Water Flow inside Polamide Reverse Osmosis Membranes: A Non-Equilibrium Molecular Dynamics Study.
Song, Yang; Xu, Fang; Wei, Mingjie; Wang, Yong
2017-02-23
Water flow inside polyamide (PA) reverse osmosis (RO) membranes is studied by steady state nonequilibrium molecular dynamics (NEMD) simulations in this work. The PA RO membrane is constructed with the all-atom model, and the density and average pore size obtained thereby are consistent with the latest experimental results. To obtain the time-independent water flux, a steady state NEMD method is used under various pressure drops. The water flux in our simulations, which is calculated under higher pressure drops, is in a linear relation with the pressure drops. Hence, the water flux in lower pressure drops can be reliably estimated, which could be compared with the experimental results. The molecular details of water flowing inside the membrane are considered. The radial distribution function and residence time of water around various groups of polyamide are introduced to analyze the water velocities around these groups, and we find that water molecules flow faster around benzene rings than around carboxyl or amino groups in the membrane, which implies that the main resistance of mass transport of water molecules comes from the carboxyl or amino groups inside the membranes. This finding is in good consistency with experimental results and suggests that less free carboxyl or amino groups should be generated inside RO membranes to enhance water permeance.
Cembran, Alessandro; Masterson, Larry R.; McClendon, Christopher L.; Taylor, Susan S.; Gao, Jiali; Veglia, Gianluigi
2013-01-01
The catalytic subunit of protein kinase A (PKA-C) is subject to several post- or co-translational modifications that regulate its activity both spatially and temporally. Among those, N-myristoylation increases the kinase affinity for membranes and might also be implicated in substrate recognition and allosteric regulation. Here, we investigated the effects of N-myristoylation on the structure, dynamics, and conformational equilibrium of PKA-C using atomistic molecular dynamics simulations. We found that the myristoyl group inserts into the hydrophobic pocket and leads to a tighter packing of the A-helix against the core of the enzyme. As a result, the A-helix conformational dynamics are reduced and its motions are more coupled with the active site. Our simulations suggest that cation-π interactions between W30, R190, and R93 are responsible for coupling these motions. Two major conformations of the myristoylated N-terminus are the most populated: a long loop (LL conformation), similar to PDB:1CMK, and a helix-turn-helix (HTH conformation), similar to PDB:4DFX, which shows stronger coupling between the conformational dynamics observed at the A-helix and active site. The HTH conformation is stabilized by S10 phosphorylation of the kinase via ionic interactions between the protonated amine of K7 and the phosphate group on S10, further enhancing the dynamic coupling to the active site. These results support a role of N-myristoylation in the allosteric regulation of PKA-C. PMID:23205665
Frentrup, Hendrik; Hart, Kyle E.; Colina, Coray M.; Müller, Erich A.
2015-01-01
We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers. PMID:25764366
Frentrup, Hendrik; Hart, Kyle E; Colina, Coray M; Müller, Erich A
2015-03-10
We study the permeation dynamics of helium and carbon dioxide through an atomistically detailed model of a polymer of intrinsic microporosity, PIM-1, via non-equilibrium molecular dynamics (NEMD) simulations. This work presents the first explicit molecular modeling of gas permeation through a high free-volume polymer sample, and it demonstrates how permeability and solubility can be obtained coherently from a single simulation. Solubilities in particular can be obtained to a very high degree of confidence and within experimental inaccuracies. Furthermore, the simulations make it possible to obtain very specific information on the diffusion dynamics of penetrant molecules and yield detailed maps of gas occupancy, which are akin to a digital tomographic scan of the polymer network. In addition to determining permeability and solubility directly from NEMD simulations, the results shed light on the permeation mechanism of the penetrant gases, suggesting that the relative openness of the microporous topology promotes the anomalous diffusion of penetrant gases, which entails a deviation from the pore hopping mechanism usually observed in gas diffusion in polymers.
Equilibrium fluctuations of liquid state static properties in a subvolume by molecular dynamics.
Heyes, D M; Dini, D; Smith, E R
2016-09-14
System property fluctuations increasingly dominate a physical process as the sampling volume decreases. The purpose of this work is to explore how the fluctuation statistics of various thermodynamic properties depend on the sampling volume, using molecular dynamics (MD) simulations. First an examination of various expressions for calculating the bulk pressure of a bulk liquid is made, which includes a decomposition of the virial expression into two terms, one of which is the Method of Planes (MOP) applied to the faces of the cubic simulation cell. Then an analysis is made of the fluctuations of local density, temperature, pressure, and shear stress as a function of sampling volume (SV). Cubic and spherical shaped SVs were used within a spatially homogeneous LJ liquid at a state point along the melting curve. It is shown that the MD-generated probability distribution functions (PDFs) of all of these properties are to a good approximation Gaussian even for SV containing only a few molecules (∼10), with the variances being inversely proportional to the SV volume, Ω. For small subvolumes the shear stress PDF fits better to a Gaussian than the pressure PDF. A new stochastic sampling technique to implement the volume averaging definition of the pressure tensor is presented, which is employed for cubic, spherical, thin cubic, and spherical shell SV. This method is more efficient for less symmetric SV shapes.
Molecular dynamics study of nanobubbles in the equilibrium Lennard-Jones fluid.
Zhukhovitskii, D I
2013-10-28
We employ a model, in which the density fluctuations in a bulk liquid are represented as presence of the clusters of molecules with the lowered number of nearest neighbors (number of bonds). The nanobubble size distribution is calculated on the basis of a close analogy between the surface part of the work of formation for a cluster and for a nanobubble. The pre-exponential factor for this distribution is related to the fluid compressibility. Estimates made for different liquids show that it can be noticeably different from that adopted in the classical nucleation theory (CNT). Molecular dynamics (MD) simulation is performed for a liquid inside a macroscopic droplet of molecules interacting via the Lennard-Jones potential plus a long-range tail. The nanobubbles are identified by clusters of bond-deficient particles with the optimum number of bonds that provide the maximum nanobubble number density and maximum resolvable nanobubble equimolar size. The results of MD simulation are in qualitatively better agreement with proposed theory than with CNT.
Equilibrium fluctuations of liquid state static properties in a subvolume by molecular dynamics
NASA Astrophysics Data System (ADS)
Heyes, D. M.; Dini, D.; Smith, E. R.
2016-09-01
System property fluctuations increasingly dominate a physical process as the sampling volume decreases. The purpose of this work is to explore how the fluctuation statistics of various thermodynamic properties depend on the sampling volume, using molecular dynamics (MD) simulations. First an examination of various expressions for calculating the bulk pressure of a bulk liquid is made, which includes a decomposition of the virial expression into two terms, one of which is the Method of Planes (MOP) applied to the faces of the cubic simulation cell. Then an analysis is made of the fluctuations of local density, temperature, pressure, and shear stress as a function of sampling volume (SV). Cubic and spherical shaped SVs were used within a spatially homogeneous LJ liquid at a state point along the melting curve. It is shown that the MD-generated probability distribution functions (PDFs) of all of these properties are to a good approximation Gaussian even for SV containing only a few molecules (˜10), with the variances being inversely proportional to the SV volume, Ω. For small subvolumes the shear stress PDF fits better to a Gaussian than the pressure PDF. A new stochastic sampling technique to implement the volume averaging definition of the pressure tensor is presented, which is employed for cubic, spherical, thin cubic, and spherical shell SV. This method is more efficient for less symmetric SV shapes.
Phonon thermal transport outside of local equilibrium in nanowires via molecular dynamics
Zhou Ya; Strachan, Alejandro
2013-03-28
We study thermal transport through Pt nanowires that bridge planar contacts as a function of wire length and vibrational frequency of the contacts. When phonons in the contacts have lower average frequencies than those in the wires thermal transport occurs under conditions away from local equilibrium with low-frequency phonons experiencing a higher thermal gradient than high-frequency ones. This results in a size-dependent increase in the effective thermal conductivity of the wire with decreasing vibrational frequencies of the contacts. The interfacial resistivity when heat flows from the wire to the contact is also size-dependent and has the same physical origin in the lack of full equilibration in short nanowires. We develop a model based on a 1D atomic chain that captures the salient physics of the MD results.
Phonon thermal transport outside of local equilibrium in nanowires via molecular dynamics.
Zhou, Ya; Strachan, Alejandro
2013-03-28
We study thermal transport through Pt nanowires that bridge planar contacts as a function of wire length and vibrational frequency of the contacts. When phonons in the contacts have lower average frequencies than those in the wires thermal transport occurs under conditions away from local equilibrium with low-frequency phonons experiencing a higher thermal gradient than high-frequency ones. This results in a size-dependent increase in the effective thermal conductivity of the wire with decreasing vibrational frequencies of the contacts. The interfacial resistivity when heat flows from the wire to the contact is also size-dependent and has the same physical origin in the lack of full equilibration in short nanowires. We develop a model based on a 1D atomic chain that captures the salient physics of the MD results.
NASA Technical Reports Server (NTRS)
Benjamin, Ilan; Pohorille, Andrew
1993-01-01
The gauche-trans isomerization reaction of 1,2-dichloroethane at the liquid-vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non-Rice-Ramsperger-Kassel-Marcus behavior due to the weak solvent-solute coupling at the water liquid-vapor interface.
NASA Technical Reports Server (NTRS)
Benjamin, Ilan; Pohorille, Andrew
1993-01-01
The gauche-trans isomerization reaction of 1,2-dichloroethane at the liquid-vapor interface of water is studied using molecular-dynamics computer simulations. The solvent bulk and surface effects on the torsional potential of mean force and on barrier recrossing dynamics are computed. The isomerization reaction involves a large change in the electric dipole moment, and as a result the trans/gauche ratio is considerably affected by the transition from the bulk solvent to the surface. Reactive flux correlation function calculations of the reaction rate reveal that deviation from the transition-state theory due to barrier recrossing is greater at the surface than in the bulk water. This suggests that the system exhibits non-Rice-Ramsperger-Kassel-Marcus behavior due to the weak solvent-solute coupling at the water liquid-vapor interface.
Guo, Dezhou; Zybin, Sergey V; An, Qi; Goddard, William A; Huang, Fenglei
2016-01-21
The combustion or detonation of reacting materials at high temperature and pressure can be characterized by the Chapman-Jouguet (CJ) state that describes the chemical equilibrium of the products at the end of the reaction zone of the detonation wave for sustained detonation. This provides the critical properties and product kinetics for input to macroscale continuum simulations of energetic materials. We propose the ReaxFF Reactive Dynamics to CJ point protocol (Rx2CJ) for predicting the CJ state parameters, providing the means to predict the performance of new materials prior to synthesis and characterization, allowing the simulation based design to be done in silico. Our Rx2CJ method is based on atomistic reactive molecular dynamics (RMD) using the QM-derived ReaxFF force field. We validate this method here by predicting the CJ point and detonation products for three typical energetic materials. We find good agreement between the predicted and experimental detonation velocities, indicating that this method can reliably predict the CJ state using modest levels of computation.
Ghatage, Dhairyashil; Tomar, Gaurav Shukla, Ratnesh K.
2015-03-28
Non-equilibrium molecular dynamics (MD) simulations require imposition of non-periodic boundary conditions (NPBCs) that seamlessly account for the effect of the truncated bulk region on the simulated MD region. Standard implementation of specular boundary conditions in such simulations results in spurious density and force fluctuations near the domain boundary and is therefore inappropriate for coupled atomistic-continuum calculations. In this work, we present a novel NPBC model that relies on boundary atoms attached to a simple cubic lattice with soft springs to account for interactions from particles which would have been present in an untruncated full domain treatment. We show that the proposed model suppresses the unphysical fluctuations in the density to less than 1% of the mean while simultaneously eliminating spurious oscillations in both mean and boundary forces. The model allows for an effective coupling of atomistic and continuum solvers as demonstrated through multiscale simulation of boundary driven singular flow in a cavity. The geometric flexibility of the model enables straightforward extension to nonplanar complex domains without any adverse effects on dynamic properties such as the diffusion coefficient.
Frolov, T; Asta, M
2012-12-07
In this work a method is proposed for computing step free energies for faceted solid-liquid interfaces based on atomistic simulations. The method is demonstrated in an application to (111) interfaces in elemental Si, modeled with the classical Stillinger-Weber potential. The approach makes use of an adiabatic trapping procedure, and involves simulations of systems with coexisting solid and liquid phases separated by faceted interfaces containing islands with different sizes, for which the corresponding equilibrium temperatures are computed. We demonstrate that the calculated coexistence temperature is strongly affected by the geometry of the interface. We find that island radius is inversely proportional to superheating, allowing us to compute the step free energy by fitting simulation data within the formalism of classical nucleation theory. The step free energy value is computed to be γ(st) = 0.103 ± 0.005 × 10(-10) J/m. The approach outlined in this work paves the way to the calculation of step free energies relevant to the solidification of faceted crystals from liquid mixtures, as encountered in nanowire growth by the vapor-liquid-solid mechanism and in alloy casting. The present work also shows that at low undercoolings the Stillinger-Weber interatomic potential for Si tends to crystallize in the wurtzite, rather than the diamond-cubic structure.
A steady-state non-equilibrium molecular dynamics approach for the study of evaporation processes
NASA Astrophysics Data System (ADS)
Zhang, Jianguo; Müller-Plathe, Florian; Yahia-Ouahmed, Méziane; Leroy, Frédéric
2013-10-01
Two non-equilibrium methods (called bubble method and splitting method, respectively) have been developed and tested to study the steady state evaporation of a droplet surrounded by its vapor, where the evaporation continuously occurs at the vapor-liquid interface while the droplet size remains constant. In the bubble method, gas molecules are continuously reinserted into a free volume (represented by a bubble) located at the centre of mass of the droplet to keep the droplet size constant. In the splitting method, a molecule close to the centre of mass of the droplet is split into two: In this way, the droplet size is also maintained during the evaporation. By additional local thermostats confined to the area of insertion, the effect of frequent insertions on properties such as density and temperature can be limited to the immediate insertion area. Perturbations are not observed in other parts of the droplet. In the end, both the bubble method and the splitting method achieve steady-state droplet evaporation. Although these methods have been developed using an isolated droplet, we anticipate that they will find a wide range of applications in the study of the evaporation of isolated films and droplets or thin films on heated substrates or under confinement. They can in principle also be used to study the steady-state of other physical processes, such as the diffusion or permeation of gas molecules or ions in a pressure gradient or a concentration gradient.
Partial molar enthalpies and reaction enthalpies from equilibrium molecular dynamics simulation
Schnell, Sondre K.; Skorpa, Ragnhild; Bedeaux, Dick; Kjelstrup, Signe; Vlugt, Thijs J. H.; Simon, Jean-Marc
2014-10-14
We present a new molecular simulation technique for determining partial molar enthalpies in mixtures of gases and liquids from single simulations, without relying on particle insertions, deletions, or identity changes. The method can also be applied to systems with chemical reactions. We demonstrate our method for binary mixtures of Weeks-Chandler-Anderson particles by comparing with conventional simulation techniques, as well as for a simple model that mimics a chemical reaction. The method considers small subsystems inside a large reservoir (i.e., the simulation box), and uses the construction of Hill to compute properties in the thermodynamic limit from small-scale fluctuations. Results obtained with the new method are in excellent agreement with those from previous methods. Especially for modeling chemical reactions, our method can be a valuable tool for determining reaction enthalpies directly from a single MD simulation.
NASA Astrophysics Data System (ADS)
Futera, Zdenek; English, Niall J.
2017-07-01
The response of water to externally applied electric fields is of central relevance in the modern world, where many extraneous electric fields are ubiquitous. Historically, the application of external fields in non-equilibrium molecular dynamics has been restricted, by and large, to relatively inexpensive, more or less sophisticated, empirical models. Here, we report long-time non-equilibrium ab initio molecular dynamics in both static and oscillating (time-dependent) external electric fields, therefore opening up a new vista in rigorous studies of electric-field effects on dynamical systems with the full arsenal of electronic-structure methods. In so doing, we apply this to liquid water with state-of-the-art non-local treatment of dispersion, and we compute a range of field effects on structural and dynamical properties, such as diffusivities and hydrogen-bond kinetics.
NASA Astrophysics Data System (ADS)
Chialvo, Ariel A.; Debenedetti, Pablo G.
1991-04-01
To date, the calculation of shear viscosity for soft-core fluids via equilibrium molecular dynamics has been done almost exclusively using the Green-Kubo formalism. The alternative mean-squared displacement approach has not been used, except for hard-sphere fluids, in which case the expression proposed by Helfand [Phys. Rev. 119, 1 (1960)] has invariably been selected. When written in the form given by McQuarrie [Statistical Mechanics (Harper & Row, New York, 1976), Chap. 21], however, the mean-squared displacement approach offers significant computational advantages over both its Green-Kubo and Helfand counterparts. In order to achieve comparable statistical significance, the number of experiments needed when using the Green-Kubo or Helfand formalisms is more than an order of magnitude higher than for the McQuarrie expression. For pairwise-additive systems with zero linear momentum, the McQuarrie method yields frame-independent shear viscosities. The hitherto unexplored McQuarrie implementation of the mean-squared displacement approach to shear-viscosity calculation thus appears superior to alternative methods currently in use.
NASA Astrophysics Data System (ADS)
Nandi, Prithwish K.; Futera, Zdenek; English, Niall J.
2016-11-01
Given the fundamental role of water in governing the biochemistry of enzymes, and in regulating their wider biological activity (e.g., by local water concentration surrounding biomolecules), the influence of extraneous electric and electromagnetic (e/m) fields thereon is of central relevance to biophysics and, more widely, biology. With the increase in levels of local and atmospheric microwave-frequency radiation present in modern life, as well as other electric-field exposure, the impact upon hydration-water layers surrounding proteins, and biomolecules generally, becomes a particularly pertinent issue. Here, we present a (non-equilibrium) molecular-dynamics-simulation study on a model protein (hen egg-white lysozyme) hydrated in water, in which we determine, inter alia, translational self-diffusivities for both hen egg-white lysozyme and its hydration layer together with relaxation dynamics of the hydrogen-bond network between the protein and its hydration-layer water molecules on a residue-per-residue basis. Crucially, we perform this analysis both above and below the dynamical-transition temperature (at ˜220 K), at 300 and 200 K, respectively, and we compare the effects of external static-electric and e/m fields with linear-response-régime (r.m.s.) intensities of 0.02 V/Å. It was found that the translational self-diffusivity of hen egg-white lysozyme and its hydration-water layer are increased substantially in static fields, primarily due to the induced electrophoretic motion, whilst the water-protein hydrogen-bond-network-rearrangement kinetics can also undergo rather striking accelerations, primarily due to the enhancement of a larger-amplitude local translational and rotational motion by charged and dipolar residues, which serves to promote hydrogen-bond breakage and re-formation kinetics. These external-field effects are particularly evident at 200 K, where they serve to induce the protein- and solvation-layer-response effects redolent of dynamical
Nandi, Prithwish K; Futera, Zdenek; English, Niall J
2016-11-28
Given the fundamental role of water in governing the biochemistry of enzymes, and in regulating their wider biological activity (e.g., by local water concentration surrounding biomolecules), the influence of extraneous electric and electromagnetic (e/m) fields thereon is of central relevance to biophysics and, more widely, biology. With the increase in levels of local and atmospheric microwave-frequency radiation present in modern life, as well as other electric-field exposure, the impact upon hydration-water layers surrounding proteins, and biomolecules generally, becomes a particularly pertinent issue. Here, we present a (non-equilibrium) molecular-dynamics-simulation study on a model protein (hen egg-white lysozyme) hydrated in water, in which we determine, inter alia, translational self-diffusivities for both hen egg-white lysozyme and its hydration layer together with relaxation dynamics of the hydrogen-bond network between the protein and its hydration-layer water molecules on a residue-per-residue basis. Crucially, we perform this analysis both above and below the dynamical-transition temperature (at ∼220 K), at 300 and 200 K, respectively, and we compare the effects of external static-electric and e/m fields with linear-response-régime (r.m.s.) intensities of 0.02 V/Å. It was found that the translational self-diffusivity of hen egg-white lysozyme and its hydration-water layer are increased substantially in static fields, primarily due to the induced electrophoretic motion, whilst the water-protein hydrogen-bond-network-rearrangement kinetics can also undergo rather striking accelerations, primarily due to the enhancement of a larger-amplitude local translational and rotational motion by charged and dipolar residues, which serves to promote hydrogen-bond breakage and re-formation kinetics. These external-field effects are particularly evident at 200 K, where they serve to induce the protein- and solvation-layer-response effects redolent of dynamical
Jolley, Kenny; Gill, Simon P.A.
2009-10-20
A method for controlling the thermal boundary conditions of non-equilibrium molecular dynamics simulations is presented. The method is simple to implement into a conventional molecular dynamics code and independent of the atomistic model employed. It works by regulating the temperature in a thermostatted boundary region by feedback control to achieve the desired temperature at the edge of an inner region where the true atomistic dynamics are retained. This is necessary to avoid intrinsic boundary effects in non-equilibrium molecular dynamics simulations. Three thermostats are investigated: the global deterministic Nose-Hoover thermostat and two local stochastic thermostats, Langevin and stadium damping. The latter thermostat is introduced to avoid the adverse reflection of phonons that occurs at an abrupt interface. The method is then extended to allow atomistic/continuum models to be thermally coupled concurrently for the analysis of large steady state and transient heat conduction problems. The effectiveness of the algorithm is demonstrated for the example of heat flow down a three-dimensional atomistic rod of uniform cross-section subjected to a variety of boundary conditions.
NASA Astrophysics Data System (ADS)
Allen, Michael P.; Brown, David; Masters, Andrew J.
1994-03-01
In a recent paper, Chialvo and Debenedetti [Phys. Rev. A 43, 4289 (1991)] consider single-particle and collective expressions due, respectively, to McQuarrie [Statistical Mechanics (Harper and Row, New York, 1976)] and Helfand [Phys. Rev. 119, 1 (1960)] for the calculation of shear viscosities in molecular-dynamics simulations. We point out an error in the discussion of origin independence in this paper, and show that the prescriptions set out in it are not related to the shear viscosity.
Non-equilibrium Dynamics of DNA Nanotubes
NASA Astrophysics Data System (ADS)
Hariadi, Rizal Fajar
Can the fundamental processes that underlie molecular biology be understood and simulated by DNA nanotechnology? The early development of DNA nanotechnology by Ned Seeman was driven by the desire to find a solution to the protein crystallization problem. Much of the later development of the field was also driven by envisioned applications in computing and nanofabrication. While the DNA nanotechnology community has assembled a versatile tool kit with which DNA nanostructures of considerable complexity can be assembled, the application of this tool kit to other areas of science and technology is still in its infancy. This dissertation reports on the construction of non-equilibrium DNA nanotube dynamic to probe molecular processes in the areas of hydrodynamics and cytoskeletal behavior. As the first example, we used DNA nanotubes as a molecular probe for elongational flow measurement in different micro-scale flow settings. The hydrodynamic flow in the vicinity of simple geometrical objects, such as a rigid DNA nanotube, is amenable to rigorous theoretical investigation. We measured the distribution of elongational flows produced in progressively more complex settings, ranging from the vicinity of an orifice in a microfluidic chamber to within a bursting bubble of Pacific ocean water. This information can be used to constrain theories on the origin of life in which replication involves a hydrodynamically driven fission process, such as the coacervate fission proposed by Oparin. A second theme of this dissertation is the bottom-up construction of a de novo artificial cytoskeleton with DNA nanotubes. The work reported here encompasses structural, locomotion, and control aspects of non-equilibrium cytoskeletal behavior. We first measured the kinetic parameters of DNA nanotube assembly and tested the accuracy of the existing polymerization models in the literature. Toward recapitulation of non-equilibrium cytoskeletal dynamics, we coupled the polymerization of DNA
Non-equilibrium dynamics from RPMD and CMD
NASA Astrophysics Data System (ADS)
Welsch, Ralph; Song, Kai; Shi, Qiang; Althorpe, Stuart C.; Miller, Thomas F.
2016-11-01
We investigate the calculation of approximate non-equilibrium quantum time correlation functions (TCFs) using two popular path-integral-based molecular dynamics methods, ring-polymer molecular dynamics (RPMD) and centroid molecular dynamics (CMD). It is shown that for the cases of a sudden vertical excitation and an initial momentum impulse, both RPMD and CMD yield non-equilibrium TCFs for linear operators that are exact for high temperatures, in the t = 0 limit, and for harmonic potentials; the subset of these conditions that are preserved for non-equilibrium TCFs of non-linear operators is also discussed. Furthermore, it is shown that for these non-equilibrium initial conditions, both methods retain the connection to Matsubara dynamics that has previously been established for equilibrium initial conditions. Comparison of non-equilibrium TCFs from RPMD and CMD to Matsubara dynamics at short times reveals the orders in time to which the methods agree. Specifically, for the position-autocorrelation function associated with sudden vertical excitation, RPMD and CMD agree with Matsubara dynamics up to O (t4) and O (t1) , respectively; for the position-autocorrelation function associated with an initial momentum impulse, RPMD and CMD agree with Matsubara dynamics up to O (t5) and O (t2) , respectively. Numerical tests using model potentials for a wide range of non-equilibrium initial conditions show that RPMD and CMD yield non-equilibrium TCFs with an accuracy that is comparable to that for equilibrium TCFs. RPMD is also used to investigate excited-state proton transfer in a system-bath model, and it is compared to numerically exact calculations performed using a recently developed version of the Liouville space hierarchical equation of motion approach; again, similar accuracy is observed for non-equilibrium and equilibrium initial conditions.
Non-equilibrium dynamics from RPMD and CMD.
Welsch, Ralph; Song, Kai; Shi, Qiang; Althorpe, Stuart C; Miller, Thomas F
2016-11-28
We investigate the calculation of approximate non-equilibrium quantum time correlation functions (TCFs) using two popular path-integral-based molecular dynamics methods, ring-polymer molecular dynamics (RPMD) and centroid molecular dynamics (CMD). It is shown that for the cases of a sudden vertical excitation and an initial momentum impulse, both RPMD and CMD yield non-equilibrium TCFs for linear operators that are exact for high temperatures, in the t = 0 limit, and for harmonic potentials; the subset of these conditions that are preserved for non-equilibrium TCFs of non-linear operators is also discussed. Furthermore, it is shown that for these non-equilibrium initial conditions, both methods retain the connection to Matsubara dynamics that has previously been established for equilibrium initial conditions. Comparison of non-equilibrium TCFs from RPMD and CMD to Matsubara dynamics at short times reveals the orders in time to which the methods agree. Specifically, for the position-autocorrelation function associated with sudden vertical excitation, RPMD and CMD agree with Matsubara dynamics up to O(t(4)) and O(t(1)), respectively; for the position-autocorrelation function associated with an initial momentum impulse, RPMD and CMD agree with Matsubara dynamics up to O(t(5)) and O(t(2)), respectively. Numerical tests using model potentials for a wide range of non-equilibrium initial conditions show that RPMD and CMD yield non-equilibrium TCFs with an accuracy that is comparable to that for equilibrium TCFs. RPMD is also used to investigate excited-state proton transfer in a system-bath model, and it is compared to numerically exact calculations performed using a recently developed version of the Liouville space hierarchical equation of motion approach; again, similar accuracy is observed for non-equilibrium and equilibrium initial conditions.
NASA Astrophysics Data System (ADS)
Burnham, Christian J.; English, Niall J.
2016-04-01
Equilibrium molecular-dynamics (MD) simulations have been performed on metastable sI and sII polymorphs of empty hydrate lattices, in addition to liquid water and ice Ih. The non-polarisable TIP4P-2005, simple point charge model (SPC), and polarisable Thole-type models (TTM): TTM2, TTM3, and TTM4 water models were used in order to survey the differences between models and to see what differences can be expected when polarisability is incorporated. Rigid and flexible variants were used of each model to gauge the effects of flexibility. Power spectra are calculated and compared to density-of-states spectra inferred from inelastic neutron scattering (INS) measurements. Thermodynamic properties were also calculated, as well as molecular-dipole distributions. It was concluded that TTM models offer optimal fidelity vis-à-vis INS spectra, together with thermodynamic properties, with the flexible TTM2 model offering optimal placement of vibrational modes.
Hafskjold, Bjørn
2017-01-01
A binary isotope mixture of Lennard-Jones/spline particles at equilibrium was perturbed by a sudden change in the system's boundary temperatures. The system's response was determined by non-equilibrium molecular dynamics (NEMD). Three transient processes were studied: 1) The propagation of a pressure (shock) wave, 2) heat diffusivity and conduction, and 3) thermal diffusion (the Ludwig-Soret effect). These three processes occur at different time scales, which makes it possible to separate them in one single NEMD run. The system was studied in liquid, supercritical, and dense gas states with various forms and strengths of the thermal perturbation. The results show that heat was initially transported by two separate mechanisms: 1) heat diffusion as described by the transient heat equation and 2) as a consequence of a pressure wave. The pressure wave travelled faster than the speed of sound, generating a shock wave in the system. Local equilibrium was found in the transient phase, even with very strong perturbations and in the shock front. Although the mass separation due to the Ludwig-Soret effect developed much slower than the pressure and temperature fields in the system at large, it was found that the Soret coefficient could be accurately determined from the initial phase of the transient and close to the heat source. This opens the possibility of a new way to analyse results from transient experiments and thereby minimize effects of gravity and convection due to buoyancy.
NASA Astrophysics Data System (ADS)
Trinh, Thuat T.; Vlugt, Thijs J. H.; Kjelstrup, Signe
2014-10-01
We report a systematic investigation of the thermal conductivity of various three-site models of carbon dioxide (CO2) using nonequilibrium molecular dynamics in the temperature range 300-1000 K and for pressures up to 200 MPa. A direct comparison with experimental data is made. Three popular CO2 force fields (MSM, EPM2, and TraPPE) and two flexible models (based on EPM2) were investigated. All rigid force fields accurately predict the equation of state for carbon dioxide for the given range of variables. They can also reproduce the thermal conductivity of CO2 at room temperature and predict a decrease of the thermal conductivity with increasing temperature. At high temperatures, the rigid models underestimate the thermal conductivity.
Trinh, Thuat T; Vlugt, Thijs J H; Kjelstrup, Signe
2014-10-07
We report a systematic investigation of the thermal conductivity of various three-site models of carbon dioxide (CO2) using nonequilibrium molecular dynamics in the temperature range 300-1000 K and for pressures up to 200 MPa. A direct comparison with experimental data is made. Three popular CO2 force fields (MSM, EPM2, and TraPPE) and two flexible models (based on EPM2) were investigated. All rigid force fields accurately predict the equation of state for carbon dioxide for the given range of variables. They can also reproduce the thermal conductivity of CO2 at room temperature and predict a decrease of the thermal conductivity with increasing temperature. At high temperatures, the rigid models underestimate the thermal conductivity.
NASA Astrophysics Data System (ADS)
Allen, Michael P.
1994-10-01
In a recent paper, Chialvo, Cummings, and Evans [Phys. Rev. E 47, 1702 (1993)] attempt to relate single-particle and collective expressions, due, respectively, to McQuarrie [Statistical Mechanics (Harper and Row, New York, 1976)] and Helfand [Phys. Rev. 119, 1 (1960)] for the calculation of shear viscosities in molecular dynamics simulations. We point out that their analysis does not correspond to the simulation algorithm they actually use, that the system-size dependence they derive and the extrapolation procedure they propose are incorrect, and that they have established no relation between their analysis and the shear viscosity. Our own analysis explains the simulation results in terms of the artificial way that periodic box boundary crossings are handled. We find no support for a link between the McQuarrie formula and any valid statistical mechanical expression for the shear viscosity.
Li, Ting; Yang, Xiaozhen; Nies, Erik
2011-01-11
The conformational properties of a finite length polyethylene chain were explored over a wide range of temperatures using a replica exchange molecular dynamics simulation providing high quality simulation data representative for the equilibrium behavior of the chain molecule. The radial distribution function (RDF) and the structure factor S(q) of the chain as a function of temperature are analyzed in detail. The different characteristic peaks in the RDF and S(q) were assigned to specific distances in the chain and structural changes occurring with the temperature. In S(q), a peak characteristic for the order in the solid state was found and used to determine the equilibrium melting temperature. A detailed scaling analysis of the structure factor covering the full q range was performed according to the work of Hammouda. In the Θ region, a quantitative analysis of the full structure factor was done using the equivalent Kuhn chain, which enabled us to assign the Θ region of our chain and to demonstrate, in our particular case, the failure of the Gaussian chain approach. The chain conformational properties at the equilibrium melting temperature are discussed using conformational distribution functions, using the largest principal component of the radius of gyration and shape parameters as order parameters. We demonstrate that for the system studied here, the Landau free energy expression based on this conformational distribution information leads to erroneous conclusions concerning the thermodynamic transition behavior. Finally, we focus on the instantaneous conformational properties at the equilibrium melting temperature and give a detailed analysis of the conformational shapes using different shape parameters and a simulation snapshot. We show that the chain does not only take the lamellar rod-like and globular conformational shapes, typical of the solid and liquid states, but can also explore many other conformational states, including the toroidal conformational
NASA Astrophysics Data System (ADS)
Bringa, E. M.; Johnson, R. E.; Dutkiewicz, Ł .
1999-05-01
Thermal spike models have been used to calculate the yields for electronic sputtering of condensed-gas solids by fast ions. In this paper molecular dynamics (MD) calculations are carried out to describe the evolution of solid Ar and O 2 following the excitation of a cylindrical track in order to test spike models. The calculated sputtering yields were found to depend linearly on the energy deposition per unit path length, d E/d x, at the highest d E/d x. This is in contrast to the spike models and the measured yields for a number of condensed-gas solids which depend quadratically on d E/d x at high d E/d x. In paper I [E.M. Bringa, R.E. Johnson, Nucl. Instr. and Meth. B 143 (1998) 513] we showed that the evolution of energy from the cylindrical track was, typically, not diffusive, as assumed in the spike models. Here we show that it is the description of the radial transport and the absence of energy transport to the surface, rather than the treatment of the ejection process, that accounts for the difference between the analytic spike models and the MD calculations. Therefore, the quadratic dependence on d E/d x of the measured sputtering yield reflects the nature of the energizing process rather than the energy transport. In this paper we describe the details of the sputtering process and compare the results here for crystalline samples to the earlier results for amorphous solids.
Oderji, Hassan Yousefi; Ding, Hongbin; Behnejad, Hassan
2011-06-01
The second self-diffusion and viscosity virial coefficients of the Lennard-Jones (LJ) fluid were calculated by a detailed evaluation of the velocity and shear-stress autocorrelation functions using equilibrium molecular dynamics simulations at low and moderate densities. Accurate calculation of these coefficients requires corresponding transport coefficient values with low degrees of uncertainty. These were obtained via very long simulations by increasing the number of particles and by using the knowledge of correlation functions in the Green-Kubo method in conjunction with their corresponding generalized Einstein relations. The values of the self-diffusion and shear viscosity coefficients have been evaluated for systems with reduced densities between 0.0005 and 0.05 and reduced temperatures from 0.7 to 30.0. This provides a new insight into the transport coefficients beyond what can be offered by the Rainwater-Friend theory, which has not been developed for the self-diffusion coefficient.
Hannah, Daniel C; Gezelter, J Daniel; Schaller, Richard D; Schatz, George C
2015-06-23
We examine the role played by surface structure and passivation in thermal transport at semiconductor/organic interfaces. Such interfaces dominate thermal transport in semiconductor nanomaterials owing to material dimensions much smaller than the bulk phonon mean free path. Utilizing reverse nonequilibrium molecular dynamics simulations, we calculate the interfacial thermal conductance (G) between a hexane solvent and chemically passivated wurtzite CdSe surfaces. In particular, we examine the dependence of G on the CdSe slab thickness, the particular exposed crystal facet, and the extent of surface passivation. Our results indicate a nonmonotonic dependence of G on ligand-grafting density, with interfaces generally exhibiting higher thermal conductance for increasing surface coverage up to ∼0.08 ligands/Å(2) (75-100% of a monolayer, depending on the particular exposed facet) and decreasing for still higher coverages. By analyzing orientational ordering and solvent penetration into the ligand layer, we show that a balance of competing effects is responsible for this nonmonotonic dependence. Although the various unpassivated CdSe surfaces exhibit similar G values, the crystal structure of an exposed facet nevertheless plays an important role in determining the interfacial thermal conductance of passivated surfaces, as the density of binding sites on a surface determines the ligand-grafting densities that may ultimately be achieved. We demonstrate that surface passivation can increase G relative to a bare surface by roughly 1 order of magnitude and that, for a given extent of passivation, thermal conductance can vary by up to a factor of ∼2 between different surfaces, suggesting that appropriately tailored nanostructures may direct heat flow in an anisotropic fashion for interface-limited thermal transport.
Tanaka, Kouichi; Ogata, Shuji; Kobayashi, Ryo; Tamura, Tomoyuki; Kitsunezuka, Masashi; Shinma, Atsushi
2013-11-21
Developing a composite material of polymers and micrometer-sized fillers with higher heat conductance is crucial to realize modular packaging of electronic components at higher densities. Enhancement mechanisms of the heat conductance of the polymer-filler interfaces by adding the surface-coupling agent in such a polymer composite material are investigated through the non-equilibrium molecular dynamics (MD) simulation. A simulation system is composed of α-alumina as the filler, bisphenol-A epoxy molecules as the polymers, and model molecules for the surface-coupling agent. The inter-atomic potential between the α-alumina and surface-coupling molecule, which is essential in the present MD simulation, is constructed to reproduce the calculated energies with the electronic density-functional theory. Through the non-equilibrium MD simulation runs, we find that the thermal resistance at the interface decreases significantly by increasing either number or lengths of the surface-coupling molecules and that the effective thermal conductivity of the system approaches to the theoretical value corresponding to zero thermal-resistance at the interface. Detailed analyses about the atomic configurations and local temperatures around the interface are performed to identify heat-transfer routes through the interface.
Intermittent many-body dynamics at equilibrium
NASA Astrophysics Data System (ADS)
Danieli, C.; Campbell, D. K.; Flach, S.
2017-06-01
The equilibrium value of an observable defines a manifold in the phase space of an ergodic and equipartitioned many-body system. A typical trajectory pierces that manifold infinitely often as time goes to infinity. We use these piercings to measure both the relaxation time of the lowest frequency eigenmode of the Fermi-Pasta-Ulam chain, as well as the fluctuations of the subsequent dynamics in equilibrium. The dynamics in equilibrium is characterized by a power-law distribution of excursion times far off equilibrium, with diverging variance. Long excursions arise from sticky dynamics close to q -breathers localized in normal mode space. Measuring the exponent allows one to predict the transition into nonergodic dynamics. We generalize our method to Klein-Gordon lattices where the sticky dynamics is due to discrete breathers localized in real space.
Zhang, Yue; Barnes, George L; Yan, Tianying; Hase, William L
2010-05-07
Model non-equilibrium molecular dynamics (MD) simulations are presented of heat transfer from a hot Au {111} substrate to an alkylthiolate self-assembled monolayer (H-SAM) to assist in obtaining an atomic-level understanding of experiments by Wang et al. (Z. Wang, J. A. Carter, A. Lagutchev, Y. K. Koh, N.-H. Seong, D. G. Cahill, and D. D. Dlott, Science, 2007, 317, 787). Different models are considered to determine how they affect the heat transfer dynamics. They include temperature equilibrated (TE) and temperature gradient (TG) thermostat models for the Au(s) surface, and soft and stiff S/Au(s) models for bonding of the S-atoms to the Au(s) surface. A detailed analysis of the non-equilibrium heat transfer at the heterogeneous interface is presented. There is a short time temperature gradient within the top layers of the Au(s) surface. The S-atoms heat rapidly, much faster than do the C-atoms in the alkylthiolate chains. A high thermal conductivity in the H-SAM, perpendicular to the interface, results in nearly identical temperatures for the CH(2) and CH(3) groups versus time. Thermal-induced disorder is analyzed for the Au(s) substrate, the S/Au(s) interface and the H-SAM. Before heat transfer occurs from the hot Au(s) substrate to the H-SAM, there is disorder at the S/Au(s) interface and within the alkylthiolate chains arising from heat-induced disorder near the surface of hot Au(s). The short-time rapid heating of the S-atoms enhances this disorder. The increasing disorder of H-SAM chains with time results from both disorder at the Au/S interface and heat transfer to the H-SAM chains.
Communication: Microphase equilibrium and assembly dynamics
NASA Astrophysics Data System (ADS)
Zhuang, Yuan; Charbonneau, Patrick
2017-09-01
Despite many attempts, ordered equilibrium microphases have yet to be obtained in experimental colloidal suspensions. The recent computation of the equilibrium phase diagram of a microscopic, particle-based microphase former [Zhuang et al., Phys. Rev. Lett. 116, 098301 (2016)] has nonetheless found such mesoscale assemblies to be thermodynamically stable. Here, we consider their equilibrium and assembly dynamics. At intermediate densities above the order-disorder transition, we identify four different dynamical regimes and the structural changes that underlie the dynamical crossovers from one disordered regime to the next. Below the order-disorder transition, we also find that periodic lamellae are the most dynamically accessible of the periodic microphases. Our analysis thus offers a comprehensive view of the dynamics of disordered microphases and a route to the assembly of periodic microphases in a putative well-controlled, experimental system.
Equilibrium molecular thermodynamics from Kirkwood sampling.
Somani, Sandeep; Okamoto, Yuko; Ballard, Andrew J; Wales, David J
2015-05-21
We present two methods for barrierless equilibrium sampling of molecular systems based on the recently proposed Kirkwood method (J. Chem. Phys. 2009, 130, 134102). Kirkwood sampling employs low-order correlations among internal coordinates of a molecule for random (or non-Markovian) sampling of the high dimensional conformational space. This is a geometrical sampling method independent of the potential energy surface. The first method is a variant of biased Monte Carlo, where Kirkwood sampling is used for generating trial Monte Carlo moves. Using this method, equilibrium distributions corresponding to different temperatures and potential energy functions can be generated from a given set of low-order correlations. Since Kirkwood samples are generated independently, this method is ideally suited for massively parallel distributed computing. The second approach is a variant of reservoir replica exchange, where Kirkwood sampling is used to construct a reservoir of conformations, which exchanges conformations with the replicas performing equilibrium sampling corresponding to different thermodynamic states. Coupling with the Kirkwood reservoir enhances sampling by facilitating global jumps in the conformational space. The efficiency of both methods depends on the overlap of the Kirkwood distribution with the target equilibrium distribution. We present proof-of-concept results for a model nine-atom linear molecule and alanine dipeptide.
Equilibrium Molecular Thermodynamics from Kirkwood Sampling
2015-01-01
We present two methods for barrierless equilibrium sampling of molecular systems based on the recently proposed Kirkwood method (J. Chem. Phys.2009, 130, 134102). Kirkwood sampling employs low-order correlations among internal coordinates of a molecule for random (or non-Markovian) sampling of the high dimensional conformational space. This is a geometrical sampling method independent of the potential energy surface. The first method is a variant of biased Monte Carlo, where Kirkwood sampling is used for generating trial Monte Carlo moves. Using this method, equilibrium distributions corresponding to different temperatures and potential energy functions can be generated from a given set of low-order correlations. Since Kirkwood samples are generated independently, this method is ideally suited for massively parallel distributed computing. The second approach is a variant of reservoir replica exchange, where Kirkwood sampling is used to construct a reservoir of conformations, which exchanges conformations with the replicas performing equilibrium sampling corresponding to different thermodynamic states. Coupling with the Kirkwood reservoir enhances sampling by facilitating global jumps in the conformational space. The efficiency of both methods depends on the overlap of the Kirkwood distribution with the target equilibrium distribution. We present proof-of-concept results for a model nine-atom linear molecule and alanine dipeptide. PMID:25915525
Instability of quantum equilibrium in Bohm's dynamics
Colin, Samuel; Valentini, Antony
2014-01-01
We consider Bohm's second-order dynamics for arbitrary initial conditions in phase space. In principle, Bohm's dynamics allows for ‘extended’ non-equilibrium, with initial momenta not equal to the gradient of phase of the wave function (as well as initial positions whose distribution departs from the Born rule). We show that extended non-equilibrium does not relax in general and is in fact unstable. This is in sharp contrast with de Broglie's first-order dynamics, for which non-standard momenta are not allowed and which shows an efficient relaxation to the Born rule for positions. On this basis, we argue that, while de Broglie's dynamics is a tenable physical theory, Bohm's dynamics is not. In a world governed by Bohm's dynamics, there would be no reason to expect to see an effective quantum theory today (even approximately), in contradiction with observation. PMID:25383020
Non-equilibrium Reaction Kinetics in Molecular Solids
NASA Astrophysics Data System (ADS)
Wood, Mitchell; Strachan, Alejandro
We explore the possibility of non-statistical chemical reactions in condense-phase energetic materials via reactive molecular dynamics (MD) simulations. We characterize the response of three unique high energy density molecular crystals to different types of insults: electric fields of various frequencies (100-4000cm-1) and strengths and direct heating at various rates. We find that non-equilibrium states can be created for short timescales when energy input targets specific vibrations through the electric fields, and that equilibration eventually occurs even while the insults remain present. Interestingly, for strong fields these relaxation timescales are comparable to those of the initial chemical decomposition of the molecules. Details of how this vibrational energy localization affects the preferred uni- or multi-molecular reactions are discussed. These results provide insight into non-equilibrium or coherent initiation of chemistry in the condensed phase that would of interest in fields ranging from catalysis to explosives.
Radiative-dynamical equilibrium states for Jupiter
NASA Technical Reports Server (NTRS)
Trafton, L. M.; Stone, P. H.
1974-01-01
In order to obtain accurate estimates of the radiative heating that drives motions in Jupiter's atmosphere, previous radiative equilibrium calculations are improved by including the NH3 opacities and updated results for the pressure-induced opacities. These additions increase the radiative lapse rate near the top of the statically unstable region and lead to a fairly constant radiative lapse rate below the tropopause. The radiative-convective equilibrium temperature structure consistent with these changes is calculated, but it differs only slightly from earlier calculations. The radiative equilibrium calculations are used to calculate whether equilibrium states can occur on Jupiter which are similar to the baroclinic instability regimes on the earth and Mars. The results show that Jupiter's dynamical regime cannot be of this kind, except possibly at very high latitudes, and that its regime must be a basically less stable one than this kind.
First-principles simulation of molecular dissociation-recombination equilibrium
NASA Astrophysics Data System (ADS)
Kylänpää, Ilkka; Rantala, Tapio T.
2011-09-01
For the first time, the equilibrium composition of chemical dissociation-recombination reaction is simulated from first-principles, only. Furthermore, beyond the conventional ab initio Born-Oppenheimer quantum chemistry the effects from the thermal and quantum equilibrium dynamics of nuclei are consistently included, as well as, the nonadiabatic coupling between the electrons and the nuclei. This has been accomplished by the path integral Monte Carlo simulations for full NVT quantum statistics of the H_3^+ ion. The molecular total energy, partition function, free energy, entropy, and heat capacity are evaluated in a large temperature range: from below room temperature to temperatures relevant for planetary atmospheric physics. Temperature and density dependent reaction balance of the molecular ion and its fragments above 4000 K is presented, and also the density dependence of thermal ionization above 10 000 K is demonstrated.
Molecular kinetic analysis of a local equilibrium Carnot cycle
NASA Astrophysics Data System (ADS)
Izumida, Yuki; Okuda, Koji
2017-07-01
We identify a velocity distribution function of ideal gas particles that is compatible with the local equilibrium assumption and the fundamental thermodynamic relation satisfying the endoreversibility. We find that this distribution is a Maxwell-Boltzmann distribution with a spatially uniform temperature and a spatially varying local center-of-mass velocity. We construct the local equilibrium Carnot cycle of an ideal gas, based on this distribution, and show that the efficiency of the present cycle is given by the endoreversible Carnot efficiency using the molecular kinetic temperatures of the gas. We also obtain an analytic expression of the efficiency at maximum power of our cycle under a small temperature difference. Our theory is also confirmed by a molecular dynamics simulation.
Non-equilibrium spatial dynamics of ecosystems.
Guichard, Frederic; Gouhier, Tarik C
2014-09-01
Ecological systems show tremendous variability across temporal and spatial scales. It is this variability that ecologists try to predict and that managers attempt to harness in order to mitigate risk. However, the foundations of ecological science and its mainstream agenda focus on equilibrium dynamics to describe the balance of nature. Despite a rich body of literature on non-equilibrium ecological dynamics, we lack a well-developed set of predictions that can relate the spatiotemporal heterogeneity of natural systems to their underlying ecological processes. We argue that ecology needs to expand its current toolbox for the study of non-equilibrium ecosystems in order to both understand and manage their spatiotemporal variability. We review current approaches and outstanding questions related to the study of spatial dynamics and its application to natural ecosystems, including the design of reserves networks. We close by emphasizing the importance of ecosystem function as a key component of a non-equilibrium ecological theory, and of spatial synchrony as a central phenomenon for its inference in natural systems.
ERIC Educational Resources Information Center
Silverberg, Lee J.; Raff, Lionel M.
2015-01-01
Thermodynamic spontaneity-equilibrium criteria require that in a single-reaction system, reactions in either the forward or reverse direction at equilibrium be nonspontaneous. Conversely, the concept of dynamic equilibrium holds that forward and reverse reactions both occur at equal rates at equilibrium to the extent allowed by kinetic…
ERIC Educational Resources Information Center
Silverberg, Lee J.; Raff, Lionel M.
2015-01-01
Thermodynamic spontaneity-equilibrium criteria require that in a single-reaction system, reactions in either the forward or reverse direction at equilibrium be nonspontaneous. Conversely, the concept of dynamic equilibrium holds that forward and reverse reactions both occur at equal rates at equilibrium to the extent allowed by kinetic…
The Dynamical Equilibrium of Galaxy Clusters
NASA Astrophysics Data System (ADS)
Carlberg, R. G.; Yee, H. K. C.; Ellingson, E.; Morris, S. L.; Abraham, R.; Gravel, P.; Pritchet, C. J.; Smecker-Hane, T.; Hartwick, F. D. A.; Hesser, J. E.; Hutchings, J. B.; Oke, J. B.
1997-02-01
If a galaxy cluster is effectively in dynamical equilibrium, then all galaxy populations within the cluster must have distributions in velocity and position that individually reflect the same underlying mass distribution, although the derived virial masses can be quite different. Specifically, within the Canadian Network for Observational Cosmology cluster sample, the virial radius of the red galaxy population is, on the average, a factor of 2.05 +/- 0.34 smaller than that of the blue population. The red galaxies also have a smaller rms velocity dispersion, a factor of 1.31 +/- 0.13 within our sample. Consequently, the virial mass calculated from the blue galaxies is 3.5 +/- 1.3 times larger than from the red galaxies. However, applying the Jeans equation of stellar hydrodynamic equilibrium to the red and blue subsamples separately gives statistically identical cluster mass profiles. This is strong evidence that these clusters are effectively equilibrium systems and therefore demonstrates empirically that the masses in the virialized region are reliably estimated using dynamical techniques.
Interactive molecular dynamics
NASA Astrophysics Data System (ADS)
Schroeder, Daniel V.
2015-03-01
Physics students now have access to interactive molecular dynamics simulations that can model and animate the motions of hundreds of particles, such as noble gas atoms, that attract each other weakly at short distances but repel strongly when pressed together. Using these simulations, students can develop an understanding of forces and motions at the molecular scale, nonideal fluids, phases of matter, thermal equilibrium, nonequilibrium states, the Boltzmann distribution, the arrow of time, and much more. This article summarizes the basic features and capabilities of such a simulation, presents a variety of student exercises using it at the introductory and intermediate levels, and describes some enhancements that can further extend its uses. A working simulation code, in html5 and javascript for running within any modern Web browser, is provided as an online supplement.
Estimating equilibrium properties from non-Hamiltonian dynamics
NASA Astrophysics Data System (ADS)
VandeVondele, Joost; Rothlisberger, Ursula
2001-11-01
We derive an expression that enables the accurate estimation of equilibrium properties using non-Hamiltonian dynamics. The major advantage of our scheme is that a time average over a single non-Hamiltonian trajectory can be employed instead of an ensemble average. Hence, it can directly be used in standard molecular dynamics simulations. The connection between non-Hamiltonian dynamics and equilibrium properties is established by assigning to the individual frames of the trajectory a weight that is based on the fluctuations of the phase space compression factor. Additionally, a simple scheme that takes into account only fluctuation of a given maximum duration is introduced to reduce the statistical error. By systematically extending the duration of the allowed fluctuations, increasingly accurate results can be obtained. Non-Hamiltonian dynamics schemes that are capable to enhance sampling efficiency are applied to two model systems in order to demonstrate the practical performance of our approach for the calculation of equilibrium free energy differences and probability density profiles.
Kwac, Kijeong; Geva, Eitan
2011-07-28
We present a mixed quantum-classical molecular dynamics study of the structure and dynamics of the hydroxyl stretch in methanol/carbon tetrachloride mixtures. One of the methanol molecules is tagged, and its hydroxyl stretch is treated quantum-mechanically, while the remaining degrees of freedom are treated classically. The adiabatic Hamiltonian of the quantum-mechanical hydroxyl is diagonalized on-the-fly to obtain the corresponding adiabatic energy levels and wave functions which depend parametrically on the instantaneous configuration of the classical degrees of freedom. The dynamics of the classical degrees of freedom are in turn affected by the quantum-mechanical state of the tagged hydroxyl stretch via the corresponding Hellmann-Feynman forces. The ability of five different force-field combinations to reproduce the experimental absorption infrared spectrum of the hydroxyl stretch is examined for different isotopomers and on a wide range of compositions. It is found that, in addition to accounting for the anharmonic nature of the hydroxyl stretch, one also has to employ polarizable force fields and account for the damping of the polarizability at short distances. The equilibrium ground-state hydrogen-bonding structure and dynamics is analyzed, and its signature on the absorption infrared spectrum of the hydroxyl stretch is investigated in detail. Five different hydroxyl stretch subpopulations are identified and spectrally assigned: monomers (α), hydrogen-bond acceptors (β), hydrogen-bond donors (γ), simultaneous hydrogen-bond donors and acceptors (δ), and simultaneous hydrogen-bond donors and double-acceptors (ε). The fundamental transition frequencies of the α and β subpopulations are found to be narrowly distributed and to overlap, thereby giving rise to a single narrow band whose intensity is significantly diminished by rotational relaxation. The fundamental transition frequency distributions of the γ, δ, and ε subpopulations are found to be
Dynamic Equilibrium Surfaces for Conical Fluid-Film Bearings
NASA Astrophysics Data System (ADS)
Koltsov, A. Y.; Korneev, A. Y.; Savin, L. A.; Li, Shengbo
2017-08-01
The paper considers the procedure of constructing curves of dynamic equilibrium and problems of applicability of the procedure of linearization of lubricant film reactions around the equilibrium point for conical fluid-film bearings.
Dynamic Protonation Equilibrium of Solvated Acetic Acid
Gu, Wei; Frigato, Tomaso; Straatsma, TP; Helms, Volkhard H.
2007-04-13
For the first time, the dynamic protonation equilibrium between an amino acid side chain analogue and bulk water as well as the diffusion properties of the excess proton were successfully reproduced through unbiased computer simulations. During a 50 ns Q-HOP MD simulation, two different regimes of proton transfer were observed. Extended phases of frequent proton swapping between acetic acid and nearby water were separated by phases where the proton freely diffuses in the simulation box until it is captured again by acetic acid. The pKa of acetic acid was calculated around 3.0 based on the relative population of protonated and deprotonated states and the diffusion coefficient of excess proton was computed from the average mean squared displacement in the simulation. Both calculated values agree well with the experimental measurements.
Punctuated equilibrium dynamics in human communications
NASA Astrophysics Data System (ADS)
Peng, Dan; Han, Xiao-Pu; Wei, Zong-Wen; Wang, Bing-Hong
2015-10-01
A minimal model based on network incorporating individual interactions is proposed to study the non-Poisson statistical properties of human behavior: individuals in system interact with their neighbors, the probability of an individual acting correlates to its activity, and all the individuals involved in action will change their activities randomly. The model reproduces varieties of spatial-temporal patterns observed in empirical studies of human daily communications, providing insight into various human activities and embracing a range of realistic social interacting systems, particularly, intriguing bimodal phenomenon. This model bridges priority queueing theory and punctuated equilibrium dynamics, and our modeling and analysis is likely to shed light on non-Poisson phenomena in many complex systems.
Molecular equilibrium with condensation. [in astrophysics
NASA Technical Reports Server (NTRS)
Sharp, C. M.; Huebner, W. F.
1990-01-01
Minimization of the Gibbs energy of formation for species of chemical elements and compounds in their gas and condensed phases determines their relative abundances in a mixture in chemical equilibrium. The procedure is more general and more powerful than previous abundance determinations in multiphase astrophysical mixtures. Some results for astrophysical equations of state are presented, and the effects of condensation on opacity are briefly indicated.
NASA Astrophysics Data System (ADS)
Panczyk, T.; Warzocha, T.; Rudzinski, W.
2007-04-01
The frequency of collisions of ideal gas molecules (argon) with a rough surface has been studied. The rough/fractal surface was created using the random deposition technique. By applying various depositions the surface roughness was controlled and, as a measure of irregularity, the fractal dimensions of the surfaces were determined. The surfaces were next immersed in ideal gas and the numbers of collisions with these surfaces were counted. The calculations were carried out using the simplified molecular dynamics simulation technique (only hard core repulsions were assumed). The calculations were performed for various ratios of gas phase atoms diameter to the surface substrate atoms diameter. The results obtained showed that the size of a gas phase atom has crucial influence on the relation between the frequency of collision and the surface fractal dimension
Path integral Liouville dynamics for thermal equilibrium systems.
Liu, Jian
2014-06-14
We show a new imaginary time path integral based method--path integral Liouville dynamics (PILD), which can be derived from the equilibrium Liouville dynamics [J. Liu and W. H. Miller, J. Chem. Phys. 134, 104101 (2011)] in the Wigner phase space. Numerical tests of PILD with the simple (white noise) Langevin thermostat have been made for two strongly anharmonic model problems. Since implementation of PILD does not request any specific form of the potential energy surface, the results suggest that PILD offers a potentially useful approach for general condensed phase molecular systems to have the two important properties: conserves the quantum canonical distribution and recovers exact thermal correlation functions (of even nonlinear operators, i.e., nonlinear functions of position or momentum operators) in the classical, high temperature, and harmonic limits.
Path integral Liouville dynamics for thermal equilibrium systems
Liu, Jian
2014-06-14
We show a new imaginary time path integral based method—path integral Liouville dynamics (PILD), which can be derived from the equilibrium Liouville dynamics [J. Liu and W. H. Miller, J. Chem. Phys. 134, 104101 (2011)] in the Wigner phase space. Numerical tests of PILD with the simple (white noise) Langevin thermostat have been made for two strongly anharmonic model problems. Since implementation of PILD does not request any specific form of the potential energy surface, the results suggest that PILD offers a potentially useful approach for general condensed phase molecular systems to have the two important properties: conserves the quantum canonical distribution and recovers exact thermal correlation functions (of even nonlinear operators, i.e., nonlinear functions of position or momentum operators) in the classical, high temperature, and harmonic limits.
SOME MODELS OF STEADY STATE DYNAMIC EQUILIBRIUM,
output is maximized, interest rate equals population growth rate and real wage equals output per worker. The second equilibrium called here a sigma...equilibrium because it depends on, peoples propensity to save, has output less than maximum and interest rate either greater than (deflationary) or less than
Matching pre-equilibrium dynamics and viscous hydrodynamics
Martinez, Mauricio; Strickland, Michael
2010-02-15
We demonstrate how to match pre-equilibrium dynamics of a 0+1-dimensional quark-gluon plasma to second-order viscous hydrodynamical evolution. The matching allows us to specify the initial values of the energy density and shear tensor at the initial time of hydrodynamical evolution as a function of the lifetime of the pre-equilibrium period. We compare two models for pre-equilibrium quark-gluon plasma, longitudinal free streaming and collisionally broadened longitudinal expansion, and present analytic formulas that can be used to fix the necessary components of the energy-momentum tensor. The resulting dynamical models can be used to assess the effect of pre-equilibrium dynamics on quark-gluon plasma observables. Additionally, we investigate the dependence of entropy production on pre-equilibrium dynamics and discuss the limitations of the standard definitions of nonequilibrium entropy.
Laner, Monika; Horta, Bruno A C; Hünenberger, Philippe H
2014-11-01
The influence of the cosolutes trehalose and methanol on the structural, dynamic and thermodynamic properties of a glycerol-1-monopalmitate (GMP) bilayer and on its main transition temperature [Formula: see text] is investigated using atomistic molecular dynamics simulations (600 ns) of a GMP bilayer patch (2 × 8 × 8 lipids) at different temperatures in the range of 302 to 338 K and considering three different cosolute concentrations. Depending on the environment and temperature, these simulations present no or a single GL[Formula: see text]LC, LC[Formula: see text]GL or LC[Formula: see text]ID transition, where LC, GL and ID are the liquid crystal, gel and interdigitated phases, respectively. The trehalose molecules form a coating layer at the bilayer surface, promote the hydrogen-bonded bridging of the lipid headgroups, preserve the interaction of the headgroups with trapped water and induce a slight lateral expansion of the bilayer in the LC phase, observations that may have implications for the phenomenon of anhydrobiosis. However, this cosolute does not affect [Formula: see text] and its dependence on hydration in the concentration range considered. On the other hand, methanol molecules intercalate between the lipid headgroups, promote a lateral expansion of the bilayer in the LC phase and induce a concentration dependent decrease of [Formula: see text], observations that may have implications for the phenomenon of anesthesia. The occurrence of an ID phase in the presence of this cosolute may be viewed as an extreme consequence of lateral expansion. The analysis of the simulations also suggests the existence of two basic conservation principles: (1) the hydrogen-bond saturation principle rests on the observation that for all species present in the different systems, the total numbers of hydrogen-bonds per molecule is essentially constant, the only factor of variability being their distribution among different partners; (2) the densest packing principle
The transformation dynamics towards equilibrium in non-equilibrium w/w/o double emulsions
NASA Astrophysics Data System (ADS)
Chao, Youchuang; Mak, Sze Yi; Shum, Ho Cheung
2016-10-01
We use a glass-based microfluidic device to generate non-equilibrium water-in-water-in-oil (w/w/o) double emulsions and study how they transform into equilibrium configurations. The method relies on using three immiscible liquids, with two of them from the phase-separated aqueous two-phase systems. We find that the transformation is accompanied by an expansion rim, while the characteristic transformation speed of the rim mainly depends on the interfacial tension between the innermost and middle phases, as well as the viscosity of the innermost phase when the middle phase is non-viscous. Remarkably, the viscosity of the outermost phase has little effect on the transformation speed. Our results account for the dynamics of non-equilibrium double emulsions towards their equilibrium structure and suggest a possibility to utilize the non-equilibrium drops to synthesize functional particles.
Equilibrium solutions for microscopic stochastic systems in population dynamics.
Lachowicz, Mirosław; Ryabukha, Tatiana
2013-06-01
The present paper deals with the problem of existence of equilibrium solutions of equations describing the general population dynamics at the microscopic level of modified Liouville equation (individually--based model) corresponding to a Markov jump process. We show the existence of factorized equilibrium solutions and discuss uniqueness. The conditions guaranteeing uniqueness or non-uniqueness are proposed under the assumption of periodic structures.
Equilibrium and dynamic pleating of a crystalline bonded network
NASA Astrophysics Data System (ADS)
Ganguly, Saswati; Nath, Parswa; Horbach, Jürgen; Sollich, Peter; Karmakar, Smarajit; Sengupta, Surajit
2017-03-01
We describe a phase transition that gives rise to structurally non-trivial states in a two-dimensional ordered network of particles connected by harmonic bonds. Monte Carlo simulations reveal that the network supports, apart from the homogeneous phase, a number of heterogeneous "pleated" phases, which can be stabilised by an external field. This field is conjugate to a global collective variable quantifying "non-affineness," i.e., the deviation of local particle displacements from local affine deformation. In the pleated phase, stress is localised in ordered rows of pleats and eliminated from the rest of the lattice. The kinetics of the phase transition is unobservably slow in molecular dynamics simulation near coexistence, due to very large free energy barriers. When the external field is increased further to lower these barriers, the network exhibits rich dynamic behaviour: it transforms into a metastable phase with the stress now localised in a disordered arrangement of pleats. The pattern of pleats shows ageing dynamics and slow relaxation to equilibrium. Our predictions may be checked by experiments on tethered colloidal solids in dynamic laser traps.
Quasi-equilibrium in glassy dynamics: an algebraic view
NASA Astrophysics Data System (ADS)
Franz, Silvio; Parisi, Giorgio
2013-02-01
We study a chain of identical glassy systems in a constrained equilibrium, where each bond of the chain is forced to remain at a preassigned distance to the previous one. We apply this description to mean-field glassy systems in the limit of a long chain where each bond is close to the previous one. We show that this construction defines a pseudo-dynamic process that in specific conditions can formally describe real relaxational dynamics for long times. In particular, in mean-field spin glass models we can recover in this way the equations of Langevin dynamics in the long time limit at the dynamical transition temperature and below. We interpret the formal identity as evidence that in these situations the configuration space is explored in a quasi-equilibrium fashion. Our general formalism, which relates dynamics to equilibrium, puts slow dynamics in a new perspective and opens the way to the computation of new dynamical quantities in glassy systems.
Equilibrium distribution of heavy quarks in fokker-planck dynamics
Walton; Rafelski
2000-01-03
We obtain an explicit generalization, within Fokker-Planck dynamics, of Einstein's relation between drag, diffusion, and the equilibrium distribution for a spatially homogeneous system, considering both the transverse and longitudinal diffusion for dimension n>1. We provide a complete characterization of the equilibrium distribution in terms of the drag and diffusion transport coefficients. We apply this analysis to charm quark dynamics in a thermal quark-gluon plasma for the case of collisional equilibration.
Polylogarithmic equilibrium treatment of molecular aggregation and critical concentrations.
Michel, Denis; Ruelle, Philippe
2017-02-15
A full equilibrium treatment of molecular aggregation is presented for prototypes of 1D and 3D aggregates, with and without nucleation. By skipping complex kinetic parameters like aggregate size-dependent diffusion, the equilibrium treatment allows us to predict directly time-independent quantities such as critical concentrations. The relationships between the macroscopic equilibrium constants for different paths are first established by statistical corrections and so as to comply with the detailed balance constraints imposed by nucleation, and the composition of the mixture resulting from homogeneous aggregation is then analyzed using a polylogarithmic function. Several critical concentrations are distinguished: the residual monomer concentration at equilibrium (RMC) and the critical nucleation concentration (CNC), which is the threshold concentration of total subunits necessary for initiating aggregation. When increasing the concentration of total subunits, the RMC converges more strongly to its asymptotic value, the equilibrium constant of depolymerization, for 3D aggregates and in the case of nucleation. The CNC moderately depends on the number of subunits in the nucleus, but sharply increases with the difference between the equilibrium constants of polymerization and nucleation. As the RMC and CNC can be numerically but not analytically determined, ansatz equations connecting them to thermodynamic parameters are proposed.
Probing the equilibrium dynamics of colloidal hard spheres above the mode-coupling glass transition.
Brambilla, G; El Masri, D; Pierno, M; Berthier, L; Cipelletti, L; Petekidis, G; Schofield, A B
2009-02-27
We use dynamic light scattering and computer simulations to study equilibrium dynamics and dynamic heterogeneity in concentrated suspensions of colloidal hard spheres. Our study covers an unprecedented density range and spans seven decades in structural relaxation time, tau(alpha0, including equilibrium measurements above phi(c), the location of the glass transition deduced from fitting our data to mode-coupling theory. Instead of falling out of equilibrium, the system remains ergodic above phi(c) and enters a new dynamical regime where tau(alpha) increases with a functional form that was not anticipated by previous experiments, while the amplitude of dynamic heterogeneity grows slower than a power law with tau(alpha), as found in molecular glass formers close to the glass transition.
García-Prieto, Francisco F; Fdez Galván, Ignacio; Aguilar, Manuel A; Martín, M Elena
2011-11-21
The ASEP/MD method has been employed for studying the solvent effect on the conformational equilibrium of the alanine dipeptide in water solution. MP2 and density functional theory (DFT) levels of theory were used and results were compared. While in gas phase cyclic structures showing intramolecular hydrogen bonds were found to be the most stable, the stability order is reversed in water solution. Intermolecular interaction with the solvent causes the predominance of extended structures as the stabilizing contacts dipeptide-water are favoured. Free-energy differences in solution were calculated and PPII, α(R), and C5 conformers were identified as the most stable at MP2 level. Experimental data from Raman and IR techniques show discrepancies about the relative abundance of α(R) y C5, our results support the Raman data. The DFT level of theory agrees with MP2 in the location and stability of PPII and α(R) forms but fails in the location of C5. MP2 results suggest the possibility of finding traces of C7eq conformer in water solution, in agreement with recent experiments.
Equilibrium and non-equilibrium dynamics simultaneously operate in the Galápagos islands.
Valente, Luis M; Phillimore, Albert B; Etienne, Rampal S
2015-08-01
Island biotas emerge from the interplay between colonisation, speciation and extinction and are often the scene of spectacular adaptive radiations. A common assumption is that insular diversity is at a dynamic equilibrium, but for remote islands, such as Hawaii or Galápagos, this idea remains untested. Here, we reconstruct the temporal accumulation of terrestrial bird species of the Galápagos using a novel phylogenetic method that estimates rates of biota assembly for an entire community. We show that species richness on the archipelago is in an ascending phase and does not tend towards equilibrium. The majority of the avifauna diversifies at a slow rate, without detectable ecological limits. However, Darwin's finches form an exception: they rapidly reach a carrying capacity and subsequently follow a coalescent-like diversification process. Together, these results suggest that avian diversity of remote islands is rising, and challenge the mutual exclusivity of the non-equilibrium and equilibrium ecological paradigms.
Equilibrium and nonequilibrium dynamics of soft sphere fluids.
Ding, Yajun; Mittal, Jeetain
2015-07-14
We use computer simulations to test the freezing-point scaling relationship between equilibrium transport coefficients (self-diffusivity, viscosity) and thermodynamic parameters for soft sphere fluids. The fluid particles interact via the inverse-power potential (IPP), and the particle softness is changed by modifying the exponent of the distance-dependent potential term. In the case of IPP fluids, density and temperature are not independent variables and can be combined to obtain a coupling parameter to define the thermodynamic state of the system. We find that the rescaled coupling parameter, based on its value at the freezing point, can approximately collapse the diffusivity and viscosity data for IPP fluids over a wide range of particle softness. Even though the collapse is far from perfect, the freezing-point scaling relationship provides a convenient and effective way to compare the structure and dynamics of fluid systems with different particle softness. We further show that an alternate scaling relationship based on two-body excess entropy can provide an almost perfect collapse of the diffusivity and viscosity data below the freezing transition. Next, we perform nonequilibrium molecular dynamics simulations to calculate the shear-dependent viscosity and to identify the distinct role of particle softness in underlying structural changes associated with rheological properties. Qualitatively, we find a similar shear-thinning behavior for IPP fluids with different particle softness, though softer particles exhibit stronger shear-thinning tendency. By investigating the distance and angle-dependent pair correlation functions in these systems, we find different structural features in the case of IPP fluids with hard-sphere like and softer particle interactions. Interestingly, shear-thinning in hard-sphere like fluids is accompanied by enhanced translational order, whereas softer fluids exhibit loss of order with shear. Our results provide a systematic evaluation
Non-equilibrium many body dynamics
Creutz, M.; Gyulassy, M.
1997-09-22
This Riken BNL Research Center Symposium on Non-Equilibrium Many Body Physics was held on September 23-25, 1997 as part of the official opening ceremony of the Center at Brookhaven National Lab. A major objective of theoretical work at the center is to elaborate on the full spectrum of strong interaction physics based on QCD, including the physics of confinement and chiral symmetry breaking, the parton structure of hadrons and nuclei, and the phenomenology of ultra-relativistic nuclear collisions related to the up-coming experiments at RHIC. The opportunities and challenges of nuclear and particle physics in this area naturally involve aspects of the many body problem common to many other fields. The aim of this symposium was to find common theoretical threads in the area of non-equilibrium physics and modern transport theories. The program consisted of invited talks on a variety topics from the fields of atomic, condensed matter, plasma, astrophysics, cosmology, and chemistry, in addition to nuclear and particle physics. Separate abstracts have been indexed into the database for contributions to this workshop.
Thermo-chemical dynamics and chemical quasi-equilibrium of plasmas in thermal non-equilibrium
NASA Astrophysics Data System (ADS)
Massot, Marc; Graille, Benjamin; Magin, Thierry E.
2011-05-01
We examine both processes of ionization by electron and heavy-particle impact in spatially uniform plasmas at rest in the absence of external forces. A singular perturbation analysis is used to study the following physical scenario, in which thermal relaxation becomes much slower than chemical reactions. First, electron-impact ionization is investigated. The dynamics of the system rapidly becomes close to a slow dynamics manifold that allows for defining a unique chemical quasi-equilibrium for two-temperature plasmas and proving that the second law of thermodynamics is satisfied. Then, all ionization reactions are taken into account simultaneously, leading to a surprising conclusion: the inner layer for short time scale (or time boundary layer) directly leads to thermal equilibrium. Global thermo-chemical equilibrium is reached within a short time scale, involving only chemical reactions, even if thermal relaxation through elastic collisions is assumed to be slow.
Thermo-chemical dynamics and chemical quasi-equilibrium of plasmas in thermal non-equilibrium
Massot, Marc; Graille, Benjamin; Magin, Thierry E.
2011-05-20
We examine both processes of ionization by electron and heavy-particle impact in spatially uniform plasmas at rest in the absence of external forces. A singular perturbation analysis is used to study the following physical scenario, in which thermal relaxation becomes much slower than chemical reactions. First, electron-impact ionization is investigated. The dynamics of the system rapidly becomes close to a slow dynamics manifold that allows for defining a unique chemical quasi-equilibrium for two-temperature plasmas and proving that the second law of thermodynamics is satisfied. Then, all ionization reactions are taken into account simultaneously, leading to a surprising conclusion: the inner layer for short time scale (or time boundary layer) directly leads to thermal equilibrium. Global thermo-chemical equilibrium is reached within a short time scale, involving only chemical reactions, even if thermal relaxation through elastic collisions is assumed to be slow.
Nonequilibrium molecular dynamics
Hoover, W.G. . Dept. of Applied Science Lawrence Livermore National Lab., CA )
1990-11-01
The development of nonequilibrium molecular dynamics is described, with emphasis on massively-parallel simulations involving the motion of millions, soon to be billions, of atoms. Corresponding continuum simulations are also discussed. 14 refs., 8 figs.
Dynamic Stability of Equilibrium Capillary Drops
NASA Astrophysics Data System (ADS)
Feldman, William M.; Kim, Inwon C.
2014-03-01
We investigate a model for contact angle motion of quasi-static capillary drops resting on a horizontal plane. We prove global in time existence and long time behavior (convergence to equilibrium) in a class of star-shaped initial data for which we show that topological changes of drops can be ruled out for all times. Our result applies to any drop which is initially star-shaped with respect to a small ball inside the drop, given that the volume of the drop is sufficiently large. For the analysis, we combine geometric arguments based on the moving-plane type method with energy dissipation methods based on the formal gradient flow structure of the problem.
Strongly Non-equilibrium Dynamics of Nanochannel Confined DNA
NASA Astrophysics Data System (ADS)
Reisner, Walter
Nanoconfined DNA exhibits a wide-range of fascinating transient and steady-state non-equilibrium phenomena. Yet, while experiment, simulation and scaling analytics are converging on a comprehensive picture regarding the equilibrium behavior of nanochannel confined DNA, non-equilibrium behavior remains largely unexplored. In particular, while the DNA extension along the nanochannel is the key observable in equilibrium experiments, in the non-equilibrium case it is necessary to measure and model not just the extension but the molecule's full time-dependent one-dimensional concentration profile. Here, we apply controlled compressive forces to a nanochannel confined molecule via a nanodozer assay, whereby an optically trapped bead is slid down the channel at a constant speed. Upon contact with the molecule, a propagating concentration ``shockwave'' develops near the bead and the molecule is dynamically compressed. This experiment, a single-molecule implementation of a macroscopic cylinder-piston apparatus, can be used to observe the molecule response over a range of forcings and benchmark theoretical description of non-equilibrium behavior. We show that the dynamic concentration profiles, including both transient and steady-state response, can be modelled via a partial differential evolution equation combining nonlinear diffusion and convection. Lastly, we present preliminary results for dynamic compression of multiple confined molecules to explore regimes of segregation and mixing for multiple chains in confinement.
Evolution of specialization under non-equilibrium population dynamics.
Nurmi, Tuomas; Parvinen, Kalle
2013-03-21
We analyze the evolution of specialization in resource utilization in a mechanistically underpinned discrete-time model using the adaptive dynamics approach. We assume two nutritionally equivalent resources that in the absence of consumers grow sigmoidally towards a resource-specific carrying capacity. The consumers use resources according to the law of mass-action with rates involving trade-off. The resulting discrete-time model for the consumer population has over-compensatory dynamics. We illuminate the way non-equilibrium population dynamics affect the evolutionary dynamics of the resource consumption rates, and show that evolution to the trimorphic coexistence of a generalist and two specialists is possible due to asynchronous non-equilibrium population dynamics of the specialists. In addition, various forms of cyclic evolutionary dynamics are possible. Furthermore, evolutionary suicide may occur even without Allee effects and demographic stochasticity.
Step-wise pulling protocols for non-equilibrium dynamics
NASA Astrophysics Data System (ADS)
Ngo, Van Anh
ensembles, which can be used to characterize non-equilibrium dynamics. Furthermore, we have applied the stepwise pulling protocols and Jarzynski's Equality to investigate the ion selectivity of potassium channels via molecular dynamics simulations. The mechanism of the potassium ion selectivity has remained poorly understood for over fifty years, although a Nobel Prize was awarded to the discovery of the molecular structure of a potassium-selective channel in 2003. In one year of performing simulations, we were able to reproduce the major results of ion selectivity accumulated in fifty years. We have been even boldly going further to propose a new model for ion selectivity based on the structural rearrangement of the selectivity filter of potassium-selective KcsA channels. This structural rearrangement has never been shown to play such a pivotal role in selecting and conducting potassium ions, but effectively rejecting sodium ions. Using the stepwise pulling protocols, we are also able to estimate conductance for ion channels, which remains elusive by using other methods. In the light of ion channels, we have also investigated how a synthetic channel of telemeric G-quadruplex conducts different types of ions. These two studies on ion selectivity not only constitute an interesting part of this dissertation, but also will enable us to further explore a new set of ion-selectivity principles. Beside the focus of my dissertation, I used million-atom molecular dynamics simulations to investigate the mechanical properties of body-centered-cubic (BCCS) and face-centered-cubic (FCCS) supercrystals of DNA-functionalized gold nanoparticles. These properties are valuable for examining whether these supercrystals can be used in gene delivery and gene therapy. The formation of such ordered supercrystals is useful to protect DNAs or RNAs from being attacked and destroyed by enzymes in cells. I also performed all-atom molecular dynamics simulations to study a pure oleic acid (OA) membrane
Collective dynamics out of thermodynamic equilibrium
NASA Astrophysics Data System (ADS)
Robb, Gordon; Politi, Antonio
2017-04-01
Thorough numerical studies reveal that spatially extended dissipative systems with long-range interactions may give rise to a large-scale dynamics. This phenomenon, which generalizes mean-field chaos, can be interpreted as a form of subtle pattern formation, where a chaotic microscopic dynamics coexists with a macroscopic irregular behavior, sustained by the spontaneous emergence of long-wavelength "hydrodynamic" modes. This regime can emerge only if the coupling is sufficiently long ranged, otherwise normal space-time chaos is observed. In Stuart-Landau oscillators a further regime is found, where the amplitude of the hydrodynamic modes exhibits an anomalous intermediate scaling between that of collective and standard space-time chaos.
Substructured multibody molecular dynamics.
Grest, Gary Stephen; Stevens, Mark Jackson; Plimpton, Steven James; Woolf, Thomas B. (Johns Hopkins University, Baltimore, MD); Lehoucq, Richard B.; Crozier, Paul Stewart; Ismail, Ahmed E.; Mukherjee, Rudranarayan M. (Rensselaer Polytechnic Institute, Troy, NY); Draganescu, Andrei I.
2006-11-01
We have enhanced our parallel molecular dynamics (MD) simulation software LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator, lammps.sandia.gov) to include many new features for accelerated simulation including articulated rigid body dynamics via coupling to the Rensselaer Polytechnic Institute code POEMS (Parallelizable Open-source Efficient Multibody Software). We use new features of the LAMMPS software package to investigate rhodopsin photoisomerization, and water model surface tension and capillary waves at the vapor-liquid interface. Finally, we motivate the recipes of MD for practitioners and researchers in numerical analysis and computational mechanics.
Non Equilibrium Transformations of Molecular Compounds Induced Mechanically
Descamps, M.; Willart, J. F.; Dudognon, E.
2006-05-05
Results clarifying the effects of mechanical milling on molecular solids are shortly reviewed. Special attention has been paid to the temperature of milling with regard to the glass transition temperature of the compounds. It is shown that decreasing the grinding temperature has for incidence to increase the amorphization tendency whereas milling above Tg produces a crystal-to-crystal transformation between polymorphic varieties. These observations contradict the usual proposition that grinding transforms the physical state only by a heating effect which induces a local melting. Equilibrium thermodynamics does not seem to be appropriate for describing the process. The driven alloys concept offers a more rational framework to interpret the effect of the milling temperature. Other results are presented which demonstrate the possibility for grinding to realize low temperature solid state alloying which offers new promising ways to stabilize amorphous molecular solids. In a second part the effect of dehydration of a molecular hydrate is described. It is shown that the rate of the dehydration process is a driving force for this other type of mechanical non equilibrium transformation.
Equilibrium and Disequilibrium Dynamics in Cobweb Models with Time Delays
NASA Astrophysics Data System (ADS)
Gori, Luca; Guerrini, Luca; Sodini, Mauro
2015-06-01
This paper aims to study price dynamics in two different continuous time cobweb models with delays close to [Hommes, 1994]. In both cases, the stationary equilibrium may be not representative of the long-term dynamics of the model, since it is possible to observe endogenous and persistent fluctuations (supercritical Hopf bifurcations) even if a deterministic context without external shocks is considered. In the model in which markets are in equilibrium every time, we show that the existence of time delays in the expectations formation mechanism may cause chaotic dynamics similar to those obtained in [Hommes, 1994] in a discrete time context. From a mathematical point of view, we apply the Poincaré-Lindstedt perturbation method to study the local dynamic properties of the models. In addition, several numerical experiments are used to investigate global properties of the systems.
Rotational transition states: relative equilibrium points in inelastic molecular collisions
NASA Astrophysics Data System (ADS)
Wiesenfeld, L.; Faure, A.; Johann, T.
2003-04-01
Transition states (TSs) are a key ingredient in the understanding of many chemical reactions. We present here a generalization of TS theory towards rotational excitation in molecular collisions, in a multi-dimensional classical Hamiltonian framework. The treatment is based on relative equilibrium, where the two colliding molecules behave as a single rotating solid object. We illustrate the theory with the help of a simple, yet meaningful, model potential for collisions between H2O and H2, which is of great importance in the astrophysical context. We show that it is the occurrence of a rotational TS that opens up the possibility of significant angular momentum transfer.
Mesoscopic non-equilibrium thermodynamic analysis of molecular motors.
Kjelstrup, S; Rubi, J M; Pagonabarraga, I; Bedeaux, D
2013-11-28
We show that the kinetics of a molecular motor fueled by ATP and operating between a deactivated and an activated state can be derived from the principles of non-equilibrium thermodynamics applied to the mesoscopic domain. The activation by ATP, the possible slip of the motor, as well as the forward stepping carrying a load are viewed as slow diffusion along a reaction coordinate. Local equilibrium is assumed in the reaction coordinate spaces, making it possible to derive the non-equilibrium thermodynamic description. Using this scheme, we find expressions for the velocity of the motor, in terms of the driving force along the spacial coordinate, and for the chemical reaction that brings about activation, in terms of the chemical potentials of the reactants and products which maintain the cycle. The second law efficiency is defined, and the velocity corresponding to maximum power is obtained for myosin movement on actin. Experimental results fitting with the description are reviewed, giving a maximum efficiency of 0.45 at a myosin headgroup velocity of 5 × 10(-7) m s(-1). The formalism allows the introduction and test of meso-level models, which may be needed to explain experiments.
NASA Astrophysics Data System (ADS)
Wu, Hao; Nüske, Feliks; Paul, Fabian; Klus, Stefan; Koltai, Péter; Noé, Frank
2017-04-01
Markov state models (MSMs) and master equation models are popular approaches to approximate molecular kinetics, equilibria, metastable states, and reaction coordinates in terms of a state space discretization usually obtained by clustering. Recently, a powerful generalization of MSMs has been introduced, the variational approach conformation dynamics/molecular kinetics (VAC) and its special case the time-lagged independent component analysis (TICA), which allow us to approximate slow collective variables and molecular kinetics by linear combinations of smooth basis functions or order parameters. While it is known how to estimate MSMs from trajectories whose starting points are not sampled from an equilibrium ensemble, this has not yet been the case for TICA and the VAC. Previous estimates from short trajectories have been strongly biased and thus not variationally optimal. Here, we employ the Koopman operator theory and the ideas from dynamic mode decomposition to extend the VAC and TICA to non-equilibrium data. The main insight is that the VAC and TICA provide a coefficient matrix that we call Koopman model, as it approximates the underlying dynamical (Koopman) operator in conjunction with the basis set used. This Koopman model can be used to compute a stationary vector to reweight the data to equilibrium. From such a Koopman-reweighted sample, equilibrium expectation values and variationally optimal reversible Koopman models can be constructed even with short simulations. The Koopman model can be used to propagate densities, and its eigenvalue decomposition provides estimates of relaxation time scales and slow collective variables for dimension reduction. Koopman models are generalizations of Markov state models, TICA, and the linear VAC and allow molecular kinetics to be described without a cluster discretization.
Molecular rheology of perfluoropolyether lubricant via nonequilibrium molecular dynamics simulation
NASA Astrophysics Data System (ADS)
Guo, Qian; Chung, Pil Seung; Chen, Haigang; Jhon, Myung S.
2006-04-01
Molecular rheology of perfluoropolyether (PFPE) systems is particularly important in designing effective lubricants that control the friction and wear in tribological applications. Using the coarse-grained, bead-spring model, equilibrium molecular dynamics based on the Langevin equation in a quiescent flow was first employed to examine the nanostructure of PFPE. Further, by integrating the modified Langevin equation and imposing the Lees-Edwards boundary condition, nonequilibrium molecular dynamics of steady shear was investigated. We observe that the shear viscosity of PFPE system depends strongly on molecular architecture (e.g., molecular weight and endgroup functionality) and external conditions (e.g., temperature and shear rate). Our study of the flow activation energy/entropy and their correlations with nanostructure visualization showed that the PFPE structure was substantially modified.
Andersen, Hans C.
2005-01-01
Supercooled liquids near the glass transition exhibit the phenomenon of heterogeneous relaxation; at any specific time, a nominally homogeneous equilibrium fluid undergoes dynamic fluctuations in its structure on a molecular distance scale with rates that are very different in different regions of the sample. Several theoretical and simulation studies have suggested a change in the nature of the dynamics of fluids as they are supercooled, leading to the concept of a dynamic crossover that is often associated with mode coupling theory. Here, we will review the use of molecular dynamics computer simulation methods to investigate heterogeneous dynamics and dynamic crossovers in models of atomic liquids. PMID:15870201
Molecular dynamics simulations.
Lindahl, Erik
2015-01-01
Molecular dynamics has evolved from a niche method mainly applicable to model systems into a cornerstone in molecular biology. It provides us with a powerful toolbox that enables us to follow and understand structure and dynamics with extreme detail-literally on scales where individual atoms can be tracked. However, with great power comes great responsibility: Simulations will not magically provide valid results, but it requires a skilled researcher. This chapter introduces you to this, and makes you aware of some potential pitfalls. We focus on the two basic and most used methods; optimizing a structure with energy minimization and simulating motion with molecular dynamics. The statistical mechanics theory is covered briefly as well as limitations, for instance the lack of quantum effects and short timescales. As a practical example, we show each step of a simulation of a small protein, including examples of hardware and software, how to obtain a starting structure, immersing it in water, and choosing good simulation parameters. You will learn how to analyze simulations in terms of structure, fluctuations, geometrical features, and how to create ray-traced movies for presentations. With modern GPU acceleration, a desktop can perform μs-scale simulations of small proteins in a day-only 15 years ago this took months on the largest supercomputer in the world. As a final exercise, we show you how to set up, perform, and interpret such a folding simulation.
Molecular dynamics simulations.
Lindahl, Erik R
2008-01-01
Molecular simulation is a very powerful toolbox in modern molecular modeling, and enables us to follow and understand structure and dynamics with extreme detail--literally on scales where motion of individual atoms can be tracked. This chapter focuses on the two most commonly used methods, namely, energy minimization and molecular dynamics, that, respectively, optimize structure and simulate the natural motion of biological macromolecules. The common theoretical framework based on statistical mechanics is covered briefly as well as limitations of the computational approach, for instance, the lack of quantum effects and limited timescales accessible. As a practical example, a full simulation of the protein lysozyme in water is described step by step, including examples of necessary hardware and software, how to obtain suitable starting molecular structures, immersing it in a solvent, choosing good simulation parameters, and energy minimization. The chapter also describes how to analyze the simulation in terms of potential energies, structural fluctuations, coordinate stability, geometrical features, and, finally, how to create beautiful ray-traced movies that can be used in presentations.
Chen, Yunjie; Roux, Benoît
2015-01-14
A family of hybrid simulation methods that combines the advantages of Monte Carlo (MC) with the strengths of classical molecular dynamics (MD) consists in carrying out short non-equilibrium MD (neMD) trajectories to generate new configurations that are subsequently accepted or rejected via an MC process. In the simplest case where a deterministic dynamic propagator is used to generate the neMD trajectories, the familiar Metropolis acceptance criterion based on the change in the total energy ΔE, min[1, exp( − βΔE)], guarantees that the hybrid algorithm will yield the equilibrium Boltzmann distribution. However, the functional form of the acceptance probability is more complex when the non-equilibrium switching process is generated via a non-deterministic stochastic dissipative propagator coupled to a heat bath. Here, we clarify the conditions under which the Metropolis criterion remains valid to rigorously yield a proper equilibrium Boltzmann distribution within hybrid neMD-MC algorithm.
Chemical equilibrium in high pressure molecular fluid mixtures
Shaw, M.S.
1993-09-01
The N{sub atoms}PT Monte Carlo simulation method has been reformulated to incorporate multiple species and chemical reactions with changes in total number of molecules. While maintaining a constant number of each type of atom, the number of molecules is changed by turning on and off the interactions of any particular position with other molecules. Chemical reactions are allowed as a correlated move of atoms to differnt molecular locations. Equilibrium chemical composition is determined as an average over the simulation along with equation of state quantities. A large set of simulations has been made with the system N{sub 2} + O{sub 2} {rightleftharpoons} NO covering a wide range in P and T. Both Hugoniot states and the CJ point have been determined and are shown to be sensitive to the potentials between unlike species.
Non-equilibrium quantum phase transition via entanglement decoherence dynamics
Lin, Yu-Chen; Yang, Pei-Yun; Zhang, Wei-Min
2016-01-01
We investigate the decoherence dynamics of continuous variable entanglement as the system-environment coupling strength varies from the weak-coupling to the strong-coupling regimes. Due to the existence of localized modes in the strong-coupling regime, the system cannot approach equilibrium with its environment, which induces a nonequilibrium quantum phase transition. We analytically solve the entanglement decoherence dynamics for an arbitrary spectral density. The nonequilibrium quantum phase transition is demonstrated as the system-environment coupling strength varies for all the Ohmic-type spectral densities. The 3-D entanglement quantum phase diagram is obtained. PMID:27713556
Non-equilibrium quantum phase transition via entanglement decoherence dynamics.
Lin, Yu-Chen; Yang, Pei-Yun; Zhang, Wei-Min
2016-10-07
We investigate the decoherence dynamics of continuous variable entanglement as the system-environment coupling strength varies from the weak-coupling to the strong-coupling regimes. Due to the existence of localized modes in the strong-coupling regime, the system cannot approach equilibrium with its environment, which induces a nonequilibrium quantum phase transition. We analytically solve the entanglement decoherence dynamics for an arbitrary spectral density. The nonequilibrium quantum phase transition is demonstrated as the system-environment coupling strength varies for all the Ohmic-type spectral densities. The 3-D entanglement quantum phase diagram is obtained.
Molecular Dynamics Analysis of a Liquid Explosive Reaction Zone
NASA Astrophysics Data System (ADS)
Soulard, L.; Crouzet, B.
2006-07-01
We present an analysis of the reaction zone of a stationary planar detonation by a equilibrium molecular dynamics method (EMD). We particularly focus on the influence of chemical characteristics such as the reactions reversibility and endothermicity. First, equilibrium and unreacted Hugoniot of the reactive system are calculated by EMD. These results are then used to predict the detonation characteristics such as the thermodynamic properties of ZND spike and the sonic point. We observe in particular the influence of the preliminary endothermic phase on the detonation velocity and its stability. The comparison between these predictions and non equilibrium molecular dynamics calculations validate the EMD method.
Pre-equilibrium dynamics and heavy-ion observables
NASA Astrophysics Data System (ADS)
Heinz, Ulrich; Liu, Jia
2016-12-01
To bracket the importance of the pre-equilibrium stage on relativistic heavy-ion collision observables, we compare simulations where it is modeled by either free-streaming partons or fluid dynamics. These cases implement the assumptions of extremely weak vs. extremely strong coupling in the initial collision stage. Accounting for flow generated in the pre-equilibrium stage, we study the sensitivity of radial, elliptic and triangular flow on the switching time when the hydrodynamic description becomes valid. Using the hybrid code iEBE-VISHNU [C. Shen, Z. Qiu, H. Song, J. Bernhard, S. Bass and U. Heinz, Comput. Phys. Commun. 199 (2016) 61] we perform a multi-parameter search, constrained by particle ratios, integrated elliptic and triangular charged hadron flow, the mean transverse momenta of pions, kaons and protons, and the second moment < pT2 > of the proton transverse momentum spectrum, to identify optimized values for the switching time τs from pre-equilibrium to hydrodynamics, the specific shear viscosity η / s, the normalization factor of the temperature-dependent specific bulk viscosity (ζ / s) (T), and the switching temperature Tsw from viscous hydrodynamics to the hadron cascade UrQMD. With the optimized parameters, we predict and compare with experiment the pT-distributions of π, K, p, Λ, Ξ and Ω yields and their elliptic flow coefficients, focusing specifically on the mass-ordering of the elliptic flow for protons and Lambda hyperons which is incorrectly described by VISHNU without pre-equilibrium flow.
Biological Implications of Dynamical Phases in Non-equilibrium Networks
NASA Astrophysics Data System (ADS)
Murugan, Arvind; Vaikuntanathan, Suriyanarayanan
2016-03-01
Biology achieves novel functions like error correction, ultra-sensitivity and accurate concentration measurement at the expense of free energy through Maxwell Demon-like mechanisms. The design principles and free energy trade-offs have been studied for a variety of such mechanisms. In this review, we emphasize a perspective based on dynamical phases that can explain commonalities shared by these mechanisms. Dynamical phases are defined by typical trajectories executed by non-equilibrium systems in the space of internal states. We find that coexistence of dynamical phases can have dramatic consequences for function vs free energy cost trade-offs. Dynamical phases can also provide an intuitive picture of the design principles behind such biological Maxwell Demons.
Multiscale reactive molecular dynamics
NASA Astrophysics Data System (ADS)
Knight, Chris; Lindberg, Gerrick E.; Voth, Gregory A.
2012-12-01
Many processes important to chemistry, materials science, and biology cannot be described without considering electronic and nuclear-level dynamics and their coupling to slower, cooperative motions of the system. These inherently multiscale problems require computationally efficient and accurate methods to converge statistical properties. In this paper, a method is presented that uses data directly from condensed phase ab initio simulations to develop reactive molecular dynamics models that do not require predefined empirical functions. Instead, the interactions used in the reactive model are expressed as linear combinations of interpolating functions that are optimized by using a linear least-squares algorithm. One notable benefit of the procedure outlined here is the capability to minimize the number of parameters requiring nonlinear optimization. The method presented can be generally applied to multiscale problems and is demonstrated by generating reactive models for the hydrated excess proton and hydroxide ion based directly on condensed phase ab initio molecular dynamics simulations. The resulting models faithfully reproduce the water-ion structural properties and diffusion constants from the ab initio simulations. Additionally, the free energy profiles for proton transfer, which is sensitive to the structural diffusion of both ions in water, are reproduced. The high fidelity of these models to ab initio simulations will permit accurate modeling of general chemical reactions in condensed phase systems with computational efficiency orders of magnitudes greater than currently possible with ab initio simulation methods, thus facilitating a proper statistical sampling of the coupling to slow, large-scale motions of the system.
Multiscale reactive molecular dynamics
Knight, Chris; Lindberg, Gerrick E.; Voth, Gregory A.
2012-01-01
Many processes important to chemistry, materials science, and biology cannot be described without considering electronic and nuclear-level dynamics and their coupling to slower, cooperative motions of the system. These inherently multiscale problems require computationally efficient and accurate methods to converge statistical properties. In this paper, a method is presented that uses data directly from condensed phase ab initio simulations to develop reactive molecular dynamics models that do not require predefined empirical functions. Instead, the interactions used in the reactive model are expressed as linear combinations of interpolating functions that are optimized by using a linear least-squares algorithm. One notable benefit of the procedure outlined here is the capability to minimize the number of parameters requiring nonlinear optimization. The method presented can be generally applied to multiscale problems and is demonstrated by generating reactive models for the hydrated excess proton and hydroxide ion based directly on condensed phase ab initio molecular dynamics simulations. The resulting models faithfully reproduce the water-ion structural properties and diffusion constants from the ab initio simulations. Additionally, the free energy profiles for proton transfer, which is sensitive to the structural diffusion of both ions in water, are reproduced. The high fidelity of these models to ab initio simulations will permit accurate modeling of general chemical reactions in condensed phase systems with computational efficiency orders of magnitudes greater than currently possible with ab initio simulation methods, thus facilitating a proper statistical sampling of the coupling to slow, large-scale motions of the system. PMID:23249062
Global dynamics of non-equilibrium gliding in animals.
Yeaton, Isaac J; Socha, John J; Ross, Shane D
2017-03-17
Gliding flight-moving horizontally downward through the air without power-has evolved in a broad diversity of taxa and serves numerous ecologically relevant functions such as predator escape, expanding foraging locations, and finding mates, and has been suggested as an evolutionary pathway to powered flight. Historically, gliding has been conceptualized using the idealized conditions of equilibrium, in which the net aerodynamic force on the glider balances its weight. While this assumption is appealing for its simplicity, recent studies of glide trajectories have shown that equilibrium gliding is not the norm for most species. Furthermore, equilibrium theory neglects the aerodynamic differences between species, as well as how a glider can modify its glide path using control. To investigate non-equilibrium glide behavior, we developed a reduced-order model of gliding that accounts for self-similarity in the equations of motion, such that the lift and drag characteristics alone determine the glide trajectory. From analysis of velocity polar diagrams of horizontal and vertical velocity from several gliding species, we find that pitch angle, the angle between the horizontal and chord line, is a control parameter that can be exploited to modulate glide angle and glide speed. Varying pitch results in changing locations of equilibrium glide configurations in the velocity polar diagram that govern passive glide dynamics. Such analyses provide a new mechanism of interspecies comparison and tools to understand experimentally-measured kinematics data and theory. In addition, this analysis suggests that the lift and drag characteristics of aerial and aquatic autonomous gliders can be engineered to passively alter glide trajectories with minimal control effort.
Vegetation ecogeomorphology, dynamic equilibrium, and disturbance: chapter 7
Hupp, Cliff R.
2013-01-01
Early ecologists understood the need to document geomorphic form and process to explain plant species distributions. Although this relationship has been acknowledged for over a century, with the exception of a few landmark papers, only the past few decades have experienced intensive research on this interdisciplinary topic. Here the authors provide a summary of the intimate relations between vegetation and geomorphic/process on hillslopes and fluvial systems. These relations are separated into systems (primarily fluvial) in dynamic equilibrium and those that are in nonequilibrium conditions including the impacts of various human disturbances affecting landforms, geomorphic processes, and interrelated, attendant vegetation patterns and processes. The authors conclude with a conceptual model of stream regime focusing on sediment deposition, erosion, and equilibrium that can be expanded to organize and predict vegetation patterns and life history strategies.
Equilibrium boundary conditions, dynamic vacuum energy, and the big bang
Klinkhamer, F. R.
2008-10-15
The near-zero value of the cosmological constant {lambda} in an equilibrium context may be due to the existence of a self-tuning relativistic vacuum variable q. Here, a cosmological nonequilibrium context is considered with a corresponding time-dependent cosmological parameter {lambda}(t) or vacuum energy density {rho}{sub V}(t). A specific model of a closed Friedmann-Robertson-Walker universe is presented, which is determined by equilibrium boundary conditions at one instant of time (t=t{sub eq}) and a particular form of vacuum-energy dynamics (d{rho}{sub V}/dt{proportional_to}{rho}{sub M}). This homogeneous and isotropic model has a standard big bang phase at early times (t<
Nash equilibrium and evolutionary dynamics in semifinalists' dilemma
NASA Astrophysics Data System (ADS)
Baek, Seung Ki; Son, Seung-Woo; Jeong, Hyeong-Chai
2015-04-01
We consider a tournament among four equally strong semifinalists. The players have to decide how much stamina to use in the semifinals, provided that the rest is available in the final and the third-place playoff. We investigate optimal strategies for allocating stamina to the successive matches when players' prizes (payoffs) are given according to the tournament results. From the basic assumption that the probability to win a match follows a nondecreasing function of stamina difference, we present symmetric Nash equilibria for general payoff structures. We find three different phases of the Nash equilibria in the payoff space. First, when the champion wins a much bigger payoff than the others, any pure strategy can constitute a Nash equilibrium as long as all four players adopt it in common. Second, when the first two places are much more valuable than the other two, the only Nash equilibrium is such that everyone uses a pure strategy investing all stamina in the semifinal. Third, when the payoff for last place is much smaller than the others, a Nash equilibrium is formed when every player adopts a mixed strategy of using all or none of its stamina in the semifinals. In a limiting case that only last place pays the penalty, this mixed-strategy profile can be proved to be a unique symmetric Nash equilibrium, at least when the winning probability follows a Heaviside step function. Moreover, by using this Heaviside step function, we study the tournament by using evolutionary replicator dynamics to obtain analytic solutions, which reproduces the corresponding Nash equilibria on the population level and gives information on dynamic aspects.
Nash equilibrium and evolutionary dynamics in semifinalists' dilemma.
Baek, Seung Ki; Son, Seung-Woo; Jeong, Hyeong-Chai
2015-04-01
We consider a tournament among four equally strong semifinalists. The players have to decide how much stamina to use in the semifinals, provided that the rest is available in the final and the third-place playoff. We investigate optimal strategies for allocating stamina to the successive matches when players' prizes (payoffs) are given according to the tournament results. From the basic assumption that the probability to win a match follows a nondecreasing function of stamina difference, we present symmetric Nash equilibria for general payoff structures. We find three different phases of the Nash equilibria in the payoff space. First, when the champion wins a much bigger payoff than the others, any pure strategy can constitute a Nash equilibrium as long as all four players adopt it in common. Second, when the first two places are much more valuable than the other two, the only Nash equilibrium is such that everyone uses a pure strategy investing all stamina in the semifinal. Third, when the payoff for last place is much smaller than the others, a Nash equilibrium is formed when every player adopts a mixed strategy of using all or none of its stamina in the semifinals. In a limiting case that only last place pays the penalty, this mixed-strategy profile can be proved to be a unique symmetric Nash equilibrium, at least when the winning probability follows a Heaviside step function. Moreover, by using this Heaviside step function, we study the tournament by using evolutionary replicator dynamics to obtain analytic solutions, which reproduces the corresponding Nash equilibria on the population level and gives information on dynamic aspects.
Testing Equilibrium Models of Molecular Gas in the Magellanic Clouds
NASA Astrophysics Data System (ADS)
Wong, Tony
We propose to study the molecular gas fractions and physical conditions of diffuse molecular clouds in the Magellanic Clouds using ultraviolet (UV) and optical absorption spectra, principally from the Far Ultraviolet Spectroscopic Explorer (FUSE) and Hubble Space Telescope (HST) archives. We will use these data to constrain the abundance of molecular hydrogen (H_2) undetectable in CO emission surveys and to test equilibrium models that seek to predict the H_2 mass fraction and the H_2/HI ratio as functions of metallicity, column density, and thermal pressure. Our approach complements HI and CO surveys by providing direct estimates of HI and H_2 column densities. For sight lines where sufficiently high resolution spectra are available, we will use the excitation of CI to determine thermal pressures, allowing us to test models that assume thermodynamic equilibrium in order to determine the HI-H_2 balance. The recently completed Spitzer Legacy surveys of the MCs provide images of PAH emission on sub-parsec scales, which may provide a means to model the distribution and small-scale clumping of gas in the vicinity of the absorption sight lines, and thus connect the absorption data with the much coarser resolution radio data. We will investigate this possibility and the implications that small-scale clumping have for comparisons with theoretical models. A preliminary analysis of the FUSE and HST data is already underway, and we present a few early results. We seek support to continue this effort over the next two years and to disseminate our results. Our methodology is novel in several respects. It includes the use of high-resolution optical spectra to derive component models for the FUSE absorption spectra, in order to derive more accurate column densities, especially for the higher J transitions of H_2 which provide key diagnostics of density and radiation field strength. Such component models will also aid in the analysis of the CI spectra. We will work to increase
Introduction to Accelerated Molecular Dynamics
Perez, Danny
2012-07-10
Molecular Dynamics is the numerical solution of the equations of motion of a set of atoms, given an interatomic potential V and some boundary and initial conditions. Molecular Dynamics is the largest scale model that gives unbiased dynamics [x(t),p(t)] in full atomistic detail. Molecular Dynamics: is simple; is 'exact' for classical dynamics (with respect to a given V); can be used to compute any (atomistic) thermodynamical or dynamical properties; naturally handles complexity -- the system does the right thing at the right time. The physics derives only from the interatomic potential.
Parametrizing coarse grained models for molecular systems at equilibrium
NASA Astrophysics Data System (ADS)
Kalligiannaki, E.; Chazirakis, A.; Tsourtis, A.; Katsoulakis, M. A.; Plecháč, P.; Harmandaris, V.
2016-10-01
Hierarchical coarse graining of atomistic molecular systems at equilibrium has been an intensive research topic over the last few decades. In this work we (a) review theoretical and numerical aspects of different parametrization methods (structural-based, force matching and relative entropy) to derive the effective interaction potential between coarse-grained particles. All methods approximate the many body potential of mean force; resulting, however, in different optimization problems. (b) We also use a reformulation of the force matching method by introducing a generalized force matching condition for the local mean force in the sense that allows the approximation of the potential of mean force under both linear and non-linear coarse graining mappings (E. Kalligiannaki, et al., J. Chem. Phys. 2015). We apply and compare these methods to: (a) a benchmark system of two isolated methane molecules; (b) methane liquid; (c) water; and (d) an alkane fluid. Differences between the effective interactions, derived from the various methods, are found that depend on the actual system under study. The results further reveal the relation of the various methods and the sensitivities that may arise in the implementation of numerical methods used in each case.
Molecular Dynamics Calculations
NASA Technical Reports Server (NTRS)
1996-01-01
The development of thermodynamics and statistical mechanics is very important in the history of physics, and it underlines the difficulty in dealing with systems involving many bodies, even if those bodies are identical. Macroscopic systems of atoms typically contain so many particles that it would be virtually impossible to follow the behavior of all of the particles involved. Therefore, the behavior of a complete system can only be described or predicted in statistical ways. Under a grant to the NASA Lewis Research Center, scientists at the Case Western Reserve University have been examining the use of modern computing techniques that may be able to investigate and find the behavior of complete systems that have a large number of particles by tracking each particle individually. This is the study of molecular dynamics. In contrast to Monte Carlo techniques, which incorporate uncertainty from the outset, molecular dynamics calculations are fully deterministic. Although it is still impossible to track, even on high-speed computers, each particle in a system of a trillion trillion particles, it has been found that such systems can be well simulated by calculating the trajectories of a few thousand particles. Modern computers and efficient computing strategies have been used to calculate the behavior of a few physical systems and are now being employed to study important problems such as supersonic flows in the laboratory and in space. In particular, an animated video (available in mpeg format--4.4 MB) was produced by Dr. M.J. Woo, now a National Research Council fellow at Lewis, and the G-VIS laboratory at Lewis. This video shows the behavior of supersonic shocks produced by pistons in enclosed cylinders by following exactly the behavior of thousands of particles. The major assumptions made were that the particles involved were hard spheres and that all collisions with the walls and with other particles were fully elastic. The animated video was voted one of two
The Impact of Kitesurfing on the Dynamic Equilibrium.
da Luz, Rafael Leonardo Ferreira; da Silva, Fernando Alves; Coertjens, Marcelo
2016-12-01
The kitesurfing athletes endure unexpected conditions in terms of the function of irregularities in the surface of the water that requires a correct proprioceptive response in order to maintain equilibrium and execute the required movements while maintaining contact with the board and the water. The objective of this work was to use the star excursion balance test to compare the dynamic equilibrium of athletes who engage in kitesurfing activities with non-athletic subjects. Fourteen kitesurfing athletes and fifteen sedentary male subjects completed three rounds of the star excursion balance test: familiarity, test one and test two. During each phase the eight directions of the test were performed three times on each leg and the maximum distance reached by the leg (cm) was measured before being divided by the length of the lower limb (%). To compare the intergroup averages, a student test t to independent samples was performed. To compare the averages across the eight directions in the same group, the repeated-measures ANOVA test was employed and to compare the averages of the right leg and the left leg, a student test t to dependent samples was used (α = 0.05). For both groups and in both legs, the distance reached in the medial, posteromedial, posterior and posterolateral directions was similar and further than the other directions. It was observed that the athletes in the comparison intergroup achieved superior results than those in the control group in the medial, posteromedial, posterior and posterolateral directions in both right and left legs and the lateral direction in the right leg (P < 0.05). Kitesurfing activities result in proportionate adaptations in the dynamic equilibrium of athletes, maybe in function of adaptations in the neuromuscular structure, resulting in a better performance in situations that cause disequilibrium.
The Impact of Kitesurfing on the Dynamic Equilibrium
da Luz, Rafael Leonardo Ferreira; da Silva, Fernando Alves; Coertjens, Marcelo
2016-01-01
Background The kitesurfing athletes endure unexpected conditions in terms of the function of irregularities in the surface of the water that requires a correct proprioceptive response in order to maintain equilibrium and execute the required movements while maintaining contact with the board and the water. Objectives The objective of this work was to use the star excursion balance test to compare the dynamic equilibrium of athletes who engage in kitesurfing activities with non-athletic subjects. Methods Fourteen kitesurfing athletes and fifteen sedentary male subjects completed three rounds of the star excursion balance test: familiarity, test one and test two. During each phase the eight directions of the test were performed three times on each leg and the maximum distance reached by the leg (cm) was measured before being divided by the length of the lower limb (%). To compare the intergroup averages, a student test t to independent samples was performed. To compare the averages across the eight directions in the same group, the repeated-measures ANOVA test was employed and to compare the averages of the right leg and the left leg, a student test t to dependent samples was used (α = 0.05). Results For both groups and in both legs, the distance reached in the medial, posteromedial, posterior and posterolateral directions was similar and further than the other directions. It was observed that the athletes in the comparison intergroup achieved superior results than those in the control group in the medial, posteromedial, posterior and posterolateral directions in both right and left legs and the lateral direction in the right leg (P < 0.05). Conclusions Kitesurfing activities result in proportionate adaptations in the dynamic equilibrium of athletes, maybe in function of adaptations in the neuromuscular structure, resulting in a better performance in situations that cause disequilibrium. PMID:28144405
Dynamic clonal equilibrium and predetermined cancer risk in Barrett's oesophagus
Martinez, Pierre; Timmer, Margriet R.; Lau, Chiu T.; Calpe, Silvia; Sancho-Serra, Maria del Carmen; Straub, Danielle; Baker, Ann-Marie; Meijer, Sybren L.; Kate, Fiebo J. W. ten; Mallant-Hent, Rosalie C.; Naber, Anton H. J.; van Oijen, Arnoud H. A. M.; Baak, Lubbertus C.; Scholten, Pieter; Böhmer, Clarisse J. M.; Fockens, Paul; Bergman, Jacques J. G. H. M.; Maley, Carlo C.; Graham, Trevor A.; Krishnadath, Kausilia K
2016-01-01
Surveillance of Barrett's oesophagus allows us to study the evolutionary dynamics of a human neoplasm over time. Here we use multicolour fluorescence in situ hybridization on brush cytology specimens, from two time points with a median interval of 37 months in 195 non-dysplastic Barrett's patients, and a third time point in a subset of 90 patients at a median interval of 36 months, to study clonal evolution at single-cell resolution. Baseline genetic diversity predicts progression and remains in a stable dynamic equilibrium over time. Clonal expansions are rare, being detected once every 36.8 patient years, and growing at an average rate of 1.58 cm2 (95% CI: 0.09–4.06) per year, often involving the p16 locus. This suggests a lack of strong clonal selection in Barrett's and that the malignant potential of ‘benign' Barrett's lesions is predetermined, with important implications for surveillance programs. PMID:27538785
NASA Astrophysics Data System (ADS)
Martin, Fernando
2015-05-01
The development of attosecond laser pulses allows one to probe the inner working of atoms, molecules and surfaces on the timescale of the electronic response. In molecules, attosecond pump-probe spectroscopy enables investigations of the prompt charge redistribution and localization that accompany photo-excitation processes, where a molecule is lifted from the ground Born-Oppenheimer potential energy surface to one or more excited surfaces, and where subsequent photochemistry evolves on femto- and attosecond timescales. In this talk I will present a few theoretical examples of realistic molecular attosecond pump-probe experiments in which simple molecules are ionized with a single attosecond pulse (or a train of attosecond pulses) and are subsequently probed by one or several infrared or xuv few-cycle pulses. The evolution of the electronic and nuclear densities in the photo-excited molecule or remaining molecular ions is calculated with attosecond time-resolution and is visualized by varying the delay between the pump and probe pulses. The results of these calculations allow us to explain several experimental observations as well as to guide future experimental efforts to uncover ultrafast electron and nuclear dynamics in molecules.
VMD: visual molecular dynamics.
Humphrey, W; Dalke, A; Schulten, K
1996-02-01
VMD is a molecular graphics program designed for the display and analysis of molecular assemblies, in particular biopolymers such as proteins and nucleic acids. VMD can simultaneously display any number of structures using a wide variety of rendering styles and coloring methods. Molecules are displayed as one or more "representations," in which each representation embodies a particular rendering method and coloring scheme for a selected subset of atoms. The atoms displayed in each representation are chosen using an extensive atom selection syntax, which includes Boolean operators and regular expressions. VMD provides a complete graphical user interface for program control, as well as a text interface using the Tcl embeddable parser to allow for complex scripts with variable substitution, control loops, and function calls. Full session logging is supported, which produces a VMD command script for later playback. High-resolution raster images of displayed molecules may be produced by generating input scripts for use by a number of photorealistic image-rendering applications. VMD has also been expressly designed with the ability to animate molecular dynamics (MD) simulation trajectories, imported either from files or from a direct connection to a running MD simulation. VMD is the visualization component of MDScope, a set of tools for interactive problem solving in structural biology, which also includes the parallel MD program NAMD, and the MDCOMM software used to connect the visualization and simulation programs. VMD is written in C++, using an object-oriented design; the program, including source code and extensive documentation, is freely available via anonymous ftp and through the World Wide Web.
Ultrafast diffraction and structural dynamics: The nature of complex molecules far from equilibrium
Ruan, Chong-Yu; Lobastov, Vladimir A.; Srinivasan, Ramesh; Goodson, Boyd M.; Ihee, Hyotcherl; Zewail, Ahmed H.
2001-01-01
Studies of molecular structures at or near their equilibrium configurations have long provided information on their geometry in terms of bond distances and angles. Far-from-equilibrium structures are relatively unknown—especially for complex systems—and generally, neither their dynamics nor their average geometries can be extrapolated from equilibrium values. For such nonequilibrium structures, vibrational amplitudes and bond distances play a central role in phenomena such as energy redistribution and chemical reactivity. Ultrafast electron diffraction, which was developed to study transient molecular structures, provides a direct method for probing the nature of complex molecules far from equilibrium. Here we present our ultrafast electron diffraction observations of transient structures for two cyclic hydrocarbons. At high internal energies of ≈4 eV, these molecules display markedly different behavior. For 1,3,5-cycloheptatriene, excitation results in the formation of hot ground-state structures with bond distances similar to those of the initial structure, but with nearly three times the average vibrational amplitude. Energy is redistributed within 5 ps, but with a negative temperature characterizing the nonequilibrium population. In contrast, the ring-opening reaction of 1,3-cyclohexadiene is shown to result in hot structures with a C—C bond distance of over 1.7 Å, which is 0.2 Å away from any expected equilibrium value. Even up to 400 ps, energy remains trapped in large-amplitude motions comprised of torsion and asymmetric stretching. These studies promise a new direction for studying structural dynamics in nonequilibrium complex systems. PMID:11404473
Emami, Fereshteh; Maeder, Marcel; Abdollahi, Hamid
2015-05-07
Thermodynamic studies of equilibrium chemical reactions linked with kinetic procedures are mostly impossible by traditional approaches. In this work, the new concept of generalized kinetic study of thermodynamic parameters is introduced for dynamic data. The examples of equilibria intertwined with kinetic chemical mechanisms include molecular charge transfer complex formation reactions, pH-dependent degradation of chemical compounds and tautomerization kinetics in micellar solutions. Model-based global analysis with the possibility of calculating and embedding the equilibrium and kinetic parameters into the fitting algorithm has allowed the complete analysis of the complex reaction mechanisms. After the fitting process, the optimal equilibrium and kinetic parameters together with an estimate of their standard deviations have been obtained. This work opens up a promising new avenue for obtaining equilibrium constants through the kinetic data analysis for the kinetic reactions that involve equilibrium processes.
Non-equilibrium dynamics of artificial quantum matter
NASA Astrophysics Data System (ADS)
Babadi, Mehrtash
The rapid progress of the field of ultracold atoms during the past two decades has set new milestones in our control over matter. By cooling dilute atomic gases and molecules to nano-Kelvin temperatures, novel quantum mechanical states of matter can be realized and studied on a table-top experimental setup while bulk matter can be tailored to faithfully simulate abstract theoretical models. Two of such models which have witnessed significant experimental and theoretical attention are (1) the two-component Fermi gas with resonant s-wave interactions, and (2) the single-component Fermi gas with dipole-dipole interactions. This thesis is devoted to studying the non-equilibrium collective dynamics of these systems using the general framework of quantum kinetic theory. We present a concise review of the utilized mathematical methods in the first two chapters, including the Schwinger-Keldysh formalism of non-equilibrium quantum fields, two-particle irreducible (2PI) effective actions and the framework of quantum kinetic theory. We study the collective dynamics of the dipolar Fermi gas in a quasi-two-dimensional optical trap in chapter 3 and provide a detailed account of its dynamical crossover from the collisionless to the hydrodynamical regime. Chapter 4 is devoted to studying the dynamics of the attractive Fermi gas in the normal phase. Starting from the self-consistent T-matrix (pairing fluctuation) approximation, we systematically derive a set of quantum kinetic equations and show that they provide a globally valid description of the dynamics of the attractive Fermi gas, ranging from the weak-coupling Fermi liquid phase to the intermediate non-Fermi liquid pairing pseudogap regime and finally the strong-coupling Bose liquid phase. The shortcomings of the self-consistent T-matrix approximation in two spatial dimensions are discussed along with a proposal to overcome its unphysical behaviors. The developed kinetic formalism is finally utilized to reproduce and
Shock and Laser Induced Non-Equilibrium Chemistry in Molecular Energetics
NASA Astrophysics Data System (ADS)
Wood, Mitchell; Cherukara, Mathew; Kober, Edward; Strachan, Alejandro
2015-06-01
In this study, we have used large scale reactive molecular dynamics (MD) simulations to study how contrasting initiation mechanisms from either shock or electromagnetic insults compare to traditional thermal initiation. We will show how insults of equal strength but different character can yield vastly different reaction profiles and thus the evolution of hot-spots. For shocked RDX (Up = 2km/s), we find that the collapse of a cylindrical 40 nm diameter pore leads to a significant amount of non-equilibrium reactions followed by the formation of a sustained deflagration wave. In contrast, a hot spot that is seeded into a statically compressed crystal with matching size and temperature will quench over the same timescale, highlighting the importance of insult type. Furthermore, MD simulations of electromagnetic insults coupled to intramolecular vibrations have shown, in some cases, mode specific initial chemistry and altered kinetics of the subsequent decomposition. By leveraging spectroscopic and chemical information gathered in our MD simulations, we have been able to identify and track non-equilibrium vibrational states of these materials and correlate them to these observed changes. Implications of insult dependent reactivity and non-equilibrium chemistry will be discussed.
Floating orbital molecular dynamics simulations.
Perlt, Eva; Brüssel, Marc; Kirchner, Barbara
2014-04-21
We introduce an alternative ab initio molecular dynamics simulation as a unification of Hartree-Fock molecular dynamics and the floating orbital approach. The general scheme of the floating orbital molecular dynamics method is presented. Moreover, a simple but sophisticated guess for the orbital centers is provided to reduce the number of electronic structure optimization steps at each molecular dynamics step. The conservation of total energy and angular momentum is investigated in order to validate the floating orbital molecular dynamics approach with and without application of the initial guess. Finally, a water monomer and a water dimer are simulated, and the influence of the orbital floating on certain properties like the dipole moment is investigated.
MDplot: Visualise Molecular Dynamics.
Margreitter, Christian; Oostenbrink, Chris
2017-05-10
The MDplot package provides plotting functions to allow for automated visualisation of molecular dynamics simulation output. It is especially useful in cases where the plot generation is rather tedious due to complex file formats or when a large number of plots are generated. The graphs that are supported range from those which are standard, such as RMsD/RMsF (root-mean-square deviation and root-mean-square fluctuation, respectively) to less standard, such as thermodynamic integration analysis and hydrogen bond monitoring over time. All told, they address many commonly used analyses. In this article, we set out the MDplot package's functions, give examples of the function calls, and show the associated plots. Plotting and data parsing is separated in all cases, i.e. the respective functions can be used independently. Thus, data manipulation and the integration of additional file formats is fairly easy. Currently, the loading functions support GROMOS, GROMACS, and AMBER file formats. Moreover, we also provide a Bash interface that allows simple embedding of MDplot into Bash scripts as the final analysis step. The package can be obtained in the latest major version from CRAN (https://cran.r-project.org/package=MDplot) or in the most recent version from the project's GitHub page at https://github.com/MDplot/MDplot, where feedback is also most welcome. MDplot is published under the GPL-3 license.
Molecular dynamics with quantum fluctuations
Georgescu, Ionut; Mandelshtam, Vladimir A.
2010-09-01
A quantum dynamics approach, called Gaussian molecular dynamics, is introduced. As in the centroid molecular dynamics, the N-body quantum system is mapped to an N-body classical system with an effective Hamiltonian arising within the variational Gaussian wave-packet approximation. The approach is exact for the harmonic oscillator and for the high-temperature limit, accurate in the short-time limit and is computationally very efficient.
NASA Astrophysics Data System (ADS)
Christopoulos, Stavros; Möller, Dominik; Cota, Roberto; Gerwers, Benedikt; Weitz, Martin
2017-02-01
The degree of thermalization of electronically excited state manifolds of an absorber can be tested via optical spectroscopy. In the thermalized-manifold case, the ratio of absorption and emission is expected to follow a universal Boltzmann-type frequency scaling, known as the Kennard-Stepanov relation. Here, we investigate absorption and emission spectral profiles of rubidium, cesium, and potassium molecular dimers in a high-pressure argon buffer-gas environment and study the effect of collisionally induced redistribution. We find that, despite the use of nonlinear excitation techniques, the ratio of absorption and emission well follows the Kennard-Stepanov scaling for a variety of molecular transitions. We conclude that the upper electronic state rovibrational manifold of the molecular gas is well in thermodynamic equilibrium. Further, we demonstrate an accurate, calibration-free determination of the gas temperature from the measured spectroscopic data.
Molecular finite-size effects in stochastic models of equilibrium chemical systems.
Cianci, Claudia; Smith, Stephen; Grima, Ramon
2016-02-28
The reaction-diffusion master equation (RDME) is a standard modelling approach for understanding stochastic and spatial chemical kinetics. An inherent assumption is that molecules are point-like. Here, we introduce the excluded volume reaction-diffusion master equation (vRDME) which takes into account volume exclusion effects on stochastic kinetics due to a finite molecular radius. We obtain an exact closed form solution of the RDME and of the vRDME for a general chemical system in equilibrium conditions. The difference between the two solutions increases with the ratio of molecular diameter to the compartment length scale. We show that an increase in the fraction of excluded space can (i) lead to deviations from the classical inverse square root law for the noise-strength, (ii) flip the skewness of the probability distribution from right to left-skewed, (iii) shift the equilibrium of bimolecular reactions so that more product molecules are formed, and (iv) strongly modulate the Fano factors and coefficients of variation. These volume exclusion effects are found to be particularly pronounced for chemical species not involved in chemical conservation laws. Finally, we show that statistics obtained using the vRDME are in good agreement with those obtained from Brownian dynamics with excluded volume interactions.
Onset of Cooperative Dynamics in an Equilibrium Glass-Forming Metallic Liquid
Jaiswal, Abhishek; O’Keeffe, Stephanie; Mills, Rebecca; ...
2016-01-22
Onset of cooperative dynamics has been observed in many molecular liquids, colloids, and granular materials in the metastable regime on approaching their respective glass or jamming transition points, and is considered to play a significant role in the emergence of the slow dynamics. However, the nature of such dynamical cooperativity remains elusive in multicomponent metallic liquids characterized by complex many-body interactions and high mixing entropy. Herein, we report evidence of onset of cooperative dynamics in an equilibrium glass-forming metallic liquid (LM601: Zr51Cu36Ni4Al9). This is revealed by deviation of the mean effective diffusion coefficient from its high-temperature Arrhenius behavior below TAmore » ≈ 1300 K, i.e., a crossover from uncorrelated dynamics above TA to landscape-influenced correlated dynamics below TA. Moreover, the onset/ crossover temperature TA in such a multicomponent bulk metallic glass-forming liquid is observed at approximately twice of its calorimetric glass transition temperature (Tg ≈ 697 K) and in its stable liquid phase, unlike many molecular liquids.« less
Onset of Cooperative Dynamics in an Equilibrium Glass-Forming Metallic Liquid
Jaiswal, Abhishek; O’Keeffe, Stephanie; Mills, Rebecca; Podlesynak, Andrey; Ehlers, Georg; Dmowski, Wojciech; Lokshin, Konstantin; Stevick, Joseph; Egami, Takeshi; Zhang, Yang
2016-01-22
Onset of cooperative dynamics has been observed in many molecular liquids, colloids, and granular materials in the metastable regime on approaching their respective glass or jamming transition points, and is considered to play a significant role in the emergence of the slow dynamics. However, the nature of such dynamical cooperativity remains elusive in multicomponent metallic liquids characterized by complex many-body interactions and high mixing entropy. Herein, we report evidence of onset of cooperative dynamics in an equilibrium glass-forming metallic liquid (LM601: Zr_{51}Cu_{36}Ni_{4}Al_{9}). This is revealed by deviation of the mean effective diffusion coefficient from its high-temperature Arrhenius behavior below T_{A} ≈ 1300 K, i.e., a crossover from uncorrelated dynamics above T_{A} to landscape-influenced correlated dynamics below T_{A}. Moreover, the onset/ crossover temperature T_{A} in such a multicomponent bulk metallic glass-forming liquid is observed at approximately twice of its calorimetric glass transition temperature (T_{g} ≈ 697 K) and in its stable liquid phase, unlike many molecular liquids.
Onset of Cooperative Dynamics in an Equilibrium Glass-Forming Metallic Liquid.
Jaiswal, Abhishek; O'Keeffe, Stephanie; Mills, Rebecca; Podlesynak, Andrey; Ehlers, Georg; Dmowski, Wojciech; Lokshin, Konstantin; Stevick, Joseph; Egami, Takeshi; Zhang, Yang
2016-02-18
Onset of cooperative dynamics has been observed in many molecular liquids, colloids, and granular materials in the metastable regime on approaching their respective glass or jamming transition points, and is considered to play a significant role in the emergence of the slow dynamics. However, the nature of such dynamical cooperativity remains elusive in multicomponent metallic liquids characterized by complex many-body interactions and high mixing entropy. Herein, we report evidence of onset of cooperative dynamics in an equilibrium glass-forming metallic liquid (LM601: Zr51Cu36Ni4Al9). This is revealed by deviation of the mean effective diffusion coefficient from its high-temperature Arrhenius behavior below TA ≈ 1300 K, i.e., a crossover from uncorrelated dynamics above TA to landscape-influenced correlated dynamics below TA. Furthermore, the onset/crossover temperature TA in such a multicomponent bulk metallic glass-forming liquid is observed at approximately twice of its calorimetric glass transition temperature (Tg ≈ 697 K) and in its stable liquid phase, unlike many molecular liquids.
Molecular dynamics studies of nanofluidics and nanomechanics
NASA Astrophysics Data System (ADS)
Lee, Ki-Ho
Developing a membrane that can successfully filter molecules such as hydrocarbons, oxygen, and carbon dioxide from gaseous mixtures is an important issue for the environmental and economic industries. This potential selectivity can be predicted from atomistic simulations of the diffusion and adsorption of gases into and within carbon nanotubes. The computational nanofluidics of hydrocarbons, oxygen, and carbon dioxide have been studied with molecular dynamics simulations in the work reported here. The interactions in the system are modeled by a classical reactive empirical bond-order potential coupled to Lennard-Jones and Coulombic potentials. The transport of gas molecules for long time periods is characterized by initial non-equilibrium states followed by equilibrium states. The non-equilibrium state is induced by the diffusive motion of gas molecules from one end of the nanotubes into the vacuum or low-pressure region at the other end of the nanotubes, and lasts until the gases are evenly distributed in the nanotubes. During the non-equilibrium state, the gas molecules move back and forth through the nanotubes. It is found that this behavior, the time needed for the attainment of equilibrium, and the molecular motions at the openings of the nanotubes are affected by the density (or pressure) of gas molecules both inside and outside of the carbon nanotubes. When the gas molecules reach the end of the nanotubes, the attractive force between the tube end and the gas molecules prevents the molecules from exiting. The mechanical properties of carbon nanotubes have extended the potential applications of nanoelectromechanical systems (HEMS) such as nano-switches, nanosensors, nano-actuators, and nano-tweezers. In this study, the bending motion from externally incident Ar atom impacts on nanotubes with one firmly-fixed end is examined with classical molecular dynamics simulations. The deformation of the carbon nanotubes in the direction perpendicular to their axis is
Dynamics of Charged Particles in an Adiabatic Thermal Beam Equilibrium
NASA Astrophysics Data System (ADS)
Chen, Chiping; Wei, Haofei
2010-11-01
Charged-particle motion is studied in the self-electric and self-magnetic fields of a well-matched, intense charged-particle beam and an applied periodic solenoidal magnetic focusing field. The beam is assumed to be in a state of adiabatic thermal equilibrium. The phase space is analyzed and compared with that of the well-known Kapchinskij-Vladimirskij (KV)-type beam equilibrium. It is found that the widths of nonlinear resonances in the adiabatic thermal beam equilibrium are narrower than those in the KV-type beam equilibrium. Numerical evidence is presented, indicating almost complete elimination of chaotic particle motion in the adiabatic thermal beam equilibrium.
Adsorption equilibrium and dynamics of gasoline vapors onto polymeric adsorbents.
Jia, Lijuan; Yu, Weihua; Long, Chao; Li, Aimin
2014-03-01
The emission of gasoline vapors is becoming a significant environmental problem especially for the population-dense area and also results in a significant economic loss. In this study, adsorption equilibrium and dynamics of gasoline vapors onto macroporous and hypercrosslinked polymeric resins at 308 K were investigated and compared with commercial activated carbon (NucharWV-A 1100). The results showed that the equilibrium and breakthrough adsorption capacities of virgin macroporous and hypercrosslinked polymeric resins were lower than virgin-activated carbon. Compared with origin adsorbents, however, the breakthrough adsorption capacities of the regenerated activated carbon for gasoline vapors decreased by 58.5 % and 61.3 % when the initial concentration of gasoline vapors were 700 and 1,400 mg/L, while those of macroporous and hypercrosslinked resins decreased by 17.4 % and 17.5 %, and 46.5 % and 45.5 %, respectively. Due to the specific bimodal property in the region of micropore (0.5-2.0 nm) and meso-macropore (30-70 nm), the regenerated hypercrosslinked polymeric resin exhibited the comparable breakthrough adsorption capacities with the regenerated activated carbon at the initial concentration of 700 mg/L, and even higher when the initial concentration of gasoline vapors was 1,400 mg/L. In addition, 90 % of relative humidity had ignorable effect on the adsorption of gasoline vapors on hypercrosslinked polymeric resin. Taken together, it is expected that hypercrosslinked polymeric adsorbent would be a promising adsorbent for the removal of gasoline vapors from gas streams.
Microcomputer Calculation of Equilibrium Constants from Molecular Parameters of Gases.
ERIC Educational Resources Information Center
Venugopalan, Mundiyath
1989-01-01
Lists a BASIC program which computes the equilibrium constant as a function of temperature. Suggests use by undergraduates taking a one-year calculus-based physical chemistry course. Notes the program provides for up to four species, typically two reactants and two products. (MVL)
NASA Astrophysics Data System (ADS)
Chung, Pil Seung; Song, Wonyup; Biegler, Lorenz T.; Jhon, Myung S.
2017-05-01
During the operation of hard disk drive (HDD), the perfluoropolyether (PFPE) lubricant experiences elastic or viscous shear/elongation deformations, which affect the performance and reliability of the HDD. Therefore, the viscoelastic responses of PFPE could provide a finger print analysis in designing optimal molecular architecture of lubricants to control the tribological phenomena. In this paper, we examine the rheological responses of PFPEs including storage (elastic) and loss (viscous) moduli (G' and G″) by monitoring the time-dependent-stress-strain relationship via non-equilibrium molecular dynamics simulations. We analyzed the rheological responses by using Cox-Merz rule, and investigated the molecular structural and thermal effects on the solid-like and liquid-like behaviors of PFPEs. The temperature dependence of the endgroup agglomeration phenomena was examined, where the functional endgroups are decoupled as the temperature increases. By analyzing the relaxation processes, the molecular rheological studies will provide the optimal lubricant selection criteria to enhance the HDD performance and reliability for the heat-assisted magnetic recording applications.
The molecular photo-cell: quantum transport and energy conversion at strong non-equilibrium.
Ajisaka, Shigeru; Žunkovič, Bojan; Dubi, Yonatan
2015-02-09
The molecular photo-cell is a single molecular donor-acceptor complex attached to electrodes and subject to external illumination. Besides the obvious relevance to molecular photo-voltaics, the molecular photo-cell is of interest being a paradigmatic example for a system that inherently operates in out-of-equilibrium conditions and typically far from the linear response regime. Moreover, this system includes electrons, phonons and photons, and environments which induce coherent and incoherent processes, making it a challenging system to address theoretically. Here, using an open quantum systems approach, we analyze the non-equilibrium transport properties and energy conversion performance of a molecular photo-cell, including both coherent and incoherent processes and treating electrons, photons, and phonons on an equal footing. We find that both the non-equilibrium conditions and decoherence play a crucial role in determining the performance of the photovoltaic conversion and the optimal energy configuration of the molecular system.
The Molecular Photo-Cell: Quantum Transport and Energy Conversion at Strong Non-Equilibrium
Ajisaka, Shigeru; Žunkovič, Bojan; Dubi, Yonatan
2015-01-01
The molecular photo-cell is a single molecular donor-acceptor complex attached to electrodes and subject to external illumination. Besides the obvious relevance to molecular photo-voltaics, the molecular photo-cell is of interest being a paradigmatic example for a system that inherently operates in out-of-equilibrium conditions and typically far from the linear response regime. Moreover, this system includes electrons, phonons and photons, and environments which induce coherent and incoherent processes, making it a challenging system to address theoretically. Here, using an open quantum systems approach, we analyze the non-equilibrium transport properties and energy conversion performance of a molecular photo-cell, including both coherent and incoherent processes and treating electrons, photons, and phonons on an equal footing. We find that both the non-equilibrium conditions and decoherence play a crucial role in determining the performance of the photovoltaic conversion and the optimal energy configuration of the molecular system. PMID:25660494
Examining the Mechanical Equilibrium of Microscopic Stresses in Molecular Simulations
NASA Astrophysics Data System (ADS)
Torres-Sánchez, Alejandro; Vanegas, Juan M.; Arroyo, Marino
2015-06-01
The microscopic stress field provides a unique connection between atomistic simulations and mechanics at the nanoscale. However, its definition remains ambiguous. Rather than a mere theoretical preoccupation, we show that this fact acutely manifests itself in local stress calculations of defective graphene, lipid bilayers, and fibrous proteins. We find that popular definitions of the microscopic stress violate the continuum statements of mechanical equilibrium, and we propose an unambiguous and physically sound definition.
Hydration dynamics in water clusters via quantum molecular dynamics simulations
Turi, László
2014-05-28
We have investigated the hydration dynamics in size selected water clusters with n = 66, 104, 200, 500, and 1000 water molecules using molecular dynamics simulations. To study the most fundamental aspects of relaxation phenomena in clusters, we choose one of the simplest, still realistic, quantum mechanically treated test solute, an excess electron. The project focuses on the time evolution of the clusters following two processes, electron attachment to neutral equilibrated water clusters and electron detachment from an equilibrated water cluster anion. The relaxation dynamics is significantly different in the two processes, most notably restoring the equilibrium final state is less effective after electron attachment. Nevertheless, in both scenarios only minor cluster size dependence is observed. Significantly different relaxation patterns characterize electron detachment for interior and surface state clusters, interior state clusters relaxing significantly faster. This observation may indicate a potential way to distinguish surface state and interior state water cluster anion isomers experimentally. A comparison of equilibrium and non-equilibrium trajectories suggests that linear response theory breaks down for electron attachment at 200 K, but the results converge to reasonable agreement at higher temperatures. Relaxation following electron detachment clearly belongs to the linear regime. Cluster relaxation was also investigated using two different computational models, one preferring cavity type interior states for the excess electron in bulk water, while the other simulating non-cavity structure. While the cavity model predicts appearance of several different hydrated electron isomers in agreement with experiment, the non-cavity model locates only cluster anions with interior excess electron distribution. The present simulations show that surface isomers computed with the cavity predicting potential show similar dynamical behavior to the interior clusters of
Dynamic equilibrium of reconstituting hematopoietic stem cell populations.
O'Quigley, John
2010-12-01
Clonal dominance in hematopoietic stem cell populations is an important question of interest but not one we can directly answer. Any estimates are based on indirect measurement. For marked populations, we can equate empirical and theoretical moments for binomial sampling, in particular we can use the well-known formula for the sampling variation of a binomial proportion. The empirical variance itself cannot always be reliably estimated and some caution is needed. We describe the difficulties here and identify ready solutions which only require appropriate use of variance-stabilizing transformations. From these we obtain estimators for the steady state, or dynamic equilibrium, of the number of hematopoietic stem cells involved in repopulating the marrow. The calculations themselves are not too involved. We give the distribution theory for the estimator as well as simple approximations for practical application. As an illustration, we rework on data recently gathered to address the question as to whether or not reconstitution of marrow grafts in the clinical setting might be considered to be oligoclonal.
Geomagnetotail dynamics: Different types of equilibriums and transitions between them
NASA Astrophysics Data System (ADS)
Kropotkin, A. P.; Domrin, V. I.
2009-04-01
Two time scales are distinguished in the geomagnetotail dynamics. The small scale ( T 1) corresponds to disturbances propagating in the tail lobes, which have a relatively strong magnetic field and low plasma density. The larger scale ( T 2) corresponds to plasma motions in the plasma sheet and has a relatively weak magnetic field and a relatively higher density. A disturbance, which is initiated by a localized burst of magnetic reconnection and appears in the geomagnetotail on the time scale T 1, generates the upset of equilibrium in the plasma sheet zones with intermediate spatial dimensions (about R E). The theoretical considerations and numerical simulation indicate that the relaxation process, which subsequently proceeds on the larger time scale ( T 2), results in the appearance of extremely thin embedded current sheets and in the generation of fast plasma flows. This process gives an effective mechanism by which the magnetic energy stored in the geomagnetotail is transformed into the plasma flow kinetic energy. Such fast flows can also generate eddy plasma motions on smaller spatial scales. On the one hand, fast MHD components of this process carry a disturbance in other plasma sheet zones, where new magnetic reconnection bursts can originate at a large distance from the zone of an initial nonlinear disturbance. As a result, new recurrent processes of relaxation originate on the T 2 time scale. Alternation originating in such a way is apparently the characteristic feature of eddy disturbances actually observed in the plasma sheet.
Molecular dynamics of silicon indentation
NASA Astrophysics Data System (ADS)
Kallman, J. S.; Hoover, W. G.; Hoover, C. G.; de Groot, A. J.; Lee, S. M.; Wooten, F.
1993-04-01
We use nonequilibrium molecular dynamics to simulate the elastic-plastic deformation of silicon under tetrahedral nanometer-sized indentors. The results are described in terms of a rate-dependent and temperature-dependent phenomenological yield strength. We follow the structural change during indentation with a computer technique that allows us to model the dynamic simulation of diffraction patterns.
NASA Astrophysics Data System (ADS)
Borah, B.; Karmakar, P. K.
2015-10-01
A theoretical model is developed to study the equilibrium electromagnetic properties of a spherically symmetric dust molecular cloud (DMC) structure on the Jeans scales of space and time. It applies a new technique based on the modified Lane-Emden equation (m-LEE) of polytropic configuration. We consider a spatially inhomogeneous distribution of the massive dust grains in hydrodynamic equilibrium in the framework of exact gravito-electrostatic pressure balancing condition. Although weak relative to the massive grains, but non-zero finite, the efficacious inertial roles of the thermal species (electrons and ions) are included. A full portrayal of the lowest-order cloud surface boundary (CSB) and associated significant parameters is numerically presented. The multi-order extremization of the m-LEE solutions specifies the CSB existence at a radial point 8.58 ×1012 m relative to the center. It is shown that the CSB gets biased negatively due to the interplay of plasma-boundary wall interaction (global) and plasma sheath-sheath coupling (local) processes. It acts as an interfacial transition layer coupling the bounded and unbounded scale-dynamics of the cloud. The geometrical patterns of the bi-scale plasma coupling are elaborately analyzed. Application of our technique to neutron stars, other observed DMCs and double layers is stressed together with possible future expansion.
The Non-Equilibrium Thermodynamics and Kinetics of Focal Adhesion Dynamics
Olberding, Joseph E.; Thouless, Michael D.; Arruda, Ellen M.; Garikipati, Krishna
2010-01-01
Background We consider a focal adhesion to be made up of molecular complexes, each consisting of a ligand, an integrin molecule, and associated plaque proteins. Free energy changes drive the binding and unbinding of these complexes and thereby controls the focal adhesion's dynamic modes of growth, treadmilling and resorption. Principal Findings We have identified a competition among four thermodynamic driving forces for focal adhesion dynamics: (i) the work done during the addition of a single molecular complex of a certain size, (ii) the chemical free energy change associated with the addition of a molecular complex, (iii) the elastic free energy change associated with deformation of focal adhesions and the cell membrane, and (iv) the work done on a molecular conformational change. We have developed a theoretical treatment of focal adhesion dynamics as a nonlinear rate process governed by a classical kinetic model. We also express the rates as being driven by out-of-equilibrium thermodynamic driving forces, and modulated by kinetics. The mechanisms governed by the above four effects allow focal adhesions to exhibit a rich variety of behavior without the need to introduce special constitutive assumptions for their response. For the reaction-limited case growth, treadmilling and resorption are all predicted by a very simple chemo-mechanical model. Treadmilling requires symmetry breaking between the ends of the focal adhesion, and is achieved by driving force (i) above. In contrast, depending on its numerical value (ii) causes symmetric growth, resorption or is neutral, (iii) causes symmetric resorption, and (iv) causes symmetric growth. These findings hold for a range of conditions: temporally-constant force or stress, and for spatially-uniform and non-uniform stress distribution over the FA. The symmetric growth mode dominates for temporally-constant stress, with a reduced treadmilling regime. Significance In addition to explaining focal adhesion dynamics, this
Molecular modelling and molecular dynamics of CFTR.
Callebaut, Isabelle; Hoffmann, Brice; Lehn, Pierre; Mornon, Jean-Paul
2017-01-01
The cystic fibrosis transmembrane conductance regulator (CFTR) protein is a member of the ATP-binding cassette (ABC) transporter superfamily that functions as an ATP-gated channel. Considerable progress has been made over the last years in the understanding of the molecular basis of the CFTR functions, as well as dysfunctions causing the common genetic disease cystic fibrosis (CF). This review provides a global overview of the theoretical studies that have been performed so far, especially molecular modelling and molecular dynamics (MD) simulations. A special emphasis is placed on the CFTR-specific evolution of an ABC transporter framework towards a channel function, as well as on the understanding of the effects of disease-causing mutations and their specific modulation. This in silico work should help structure-based drug discovery and design, with a view to develop CFTR-specific pharmacotherapeutic approaches for the treatment of CF in the context of precision medicine.
Molecular Biodynamers: Dynamic Covalent Analogues of Biopolymers
2017-01-01
, molecular biodynamers are commonly produced in aqueous media under mild or even physiological conditions to suit their biorelated applications. In contrast to static biopolymers emphasizing structural stability and unity by using irreversible covalent bonds, molecular biodynamers are seeking relative structural adaptability and diversity through the formation of reversible covalent bonds. Based on these considerations, molecular biodynamers are capable of reorganizing their monomers, generating, identifying, and amplifying the fittest structures in response to environmental factors. Hence, molecular biodynamers have received considerable research attention over the past decades. Accordingly, the construction of molecular biodynamers through equilibrium polymerization of nucleobase-, carbohydrate- or amino-acid-based monomers can lead to the fabrication of dynamic analogues of nucleic acids (DyNAs), polysaccharides (glycodynamers), or proteins (dynamic proteoids), respectively. In this Account, we summarize recent advances in developing different types of molecular biodynamers as structural or functional biomimetics of biopolymers, including DyNAs, glycodynamers, and dynamic proteoids. We introduce how chemists utilize various reversible reactions to generate molecular biodynamers with specific sequences and well-ordered structures in aqueous medium. We also discuss and list their potential applications in various research fields, such as drug delivery, drug discovery, gene sensing, cancer diagnosis, and treatment. PMID:28169527
Molecular Biodynamers: Dynamic Covalent Analogues of Biopolymers.
Liu, Yun; Lehn, Jean-Marie; Hirsch, Anna K H
2017-02-21
biodynamers are commonly produced in aqueous media under mild or even physiological conditions to suit their biorelated applications. In contrast to static biopolymers emphasizing structural stability and unity by using irreversible covalent bonds, molecular biodynamers are seeking relative structural adaptability and diversity through the formation of reversible covalent bonds. Based on these considerations, molecular biodynamers are capable of reorganizing their monomers, generating, identifying, and amplifying the fittest structures in response to environmental factors. Hence, molecular biodynamers have received considerable research attention over the past decades. Accordingly, the construction of molecular biodynamers through equilibrium polymerization of nucleobase-, carbohydrate- or amino-acid-based monomers can lead to the fabrication of dynamic analogues of nucleic acids (DyNAs), polysaccharides (glycodynamers), or proteins (dynamic proteoids), respectively. In this Account, we summarize recent advances in developing different types of molecular biodynamers as structural or functional biomimetics of biopolymers, including DyNAs, glycodynamers, and dynamic proteoids. We introduce how chemists utilize various reversible reactions to generate molecular biodynamers with specific sequences and well-ordered structures in aqueous medium. We also discuss and list their potential applications in various research fields, such as drug delivery, drug discovery, gene sensing, cancer diagnosis, and treatment.
Control-volume representation of molecular dynamics.
Smith, E R; Heyes, D M; Dini, D; Zaki, T A
2012-05-01
A molecular dynamics (MD) parallel to the control volume (CV) formulation of fluid mechanics is developed by integrating the formulas of Irving and Kirkwood [J. Chem. Phys. 18, 817 (1950)] over a finite cubic volume of molecular dimensions. The Lagrangian molecular system is expressed in terms of an Eulerian CV, which yields an equivalent to Reynolds' transport theorem for the discrete system. This approach casts the dynamics of the molecular system into a form that can be readily compared to the continuum equations. The MD equations of motion are reinterpreted in terms of a Lagrangian-to-control-volume (LCV) conversion function ϑ(i) for each molecule i. The LCV function and its spatial derivatives are used to express fluxes and relevant forces across the control surfaces. The relationship between the local pressures computed using the volume average [Lutsko, J. Appl. Phys. 64, 1152 (1988)] techniques and the method of planes [Todd et al., Phys. Rev. E 52, 1627 (1995)] emerges naturally from the treatment. Numerical experiments using the MD CV method are reported for equilibrium and nonequilibrium (start-up Couette flow) model liquids, which demonstrate the advantages of the formulation. The CV formulation of the MD is shown to be exactly conservative and is, therefore, ideally suited to obtain macroscopic properties from a discrete system.
Probing equilibrium by nonequilibrium dynamics: Aging in Co/Cr superlattices
NASA Astrophysics Data System (ADS)
Binek, Christian
2013-03-01
Magnetic aging phenomena are investigated in a structurally ordered Co/Cr superlattice through measurements of magnetization relaxation, magnetic susceptibility, and hysteresis at various temperatures above and below the onset of collective magnetic order. We take advantage of the fact that controlled growth of magnetic multilayer thin films via molecular beam epitaxy allows tailoring the intra and inter-layer exchange interaction and thus enables tuning of magnetic properties including the spin-fluctuation spectra. Tailored nanoscale periodicity in Co/Cr multilayers creates mesoscopic spatial magnetic correlations with slow relaxation dynamics when quenching the system into a nonequilibrium state. Magnetization relaxation in weakly correlated spin systems depends on the microscopic spin-flip time of about 10 ns and is therefore a fast process. The spin correlations in our Co/Cr superlattice bring the magnetization dynamics to experimentally better accessible time scales of seconds or hours. In contrast to spin-glasses, where slow dynamics due to disorder and frustration is a well-known phenomenon, we tune and increase relaxation times in ordered structures. This is achieved by increasing spin-spin correlation between mesoscopically correlated regions rather than individual atomic spins, a concept with some similarity to block spin renormalization. Magnetization transients are measured after exposing the Co/Cr heterostructure to a magnetic set field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in nonequilibrium properties. Our findings suggest that
Modeling Molecular Dynamics from Simulations
Hinrichs, Nina Singhal
2009-01-28
Many important processes in biology occur at the molecular scale. A detailed understanding of these processes can lead to significant advances in the medical and life sciences. For example, many diseases are caused by protein aggregation or misfolding. One approach to studying these systems is to use physically-based computational simulations to model the interactions and movement of the molecules. While molecular simulations are computationally expensive, it is now possible to simulate many independent molecular dynamics trajectories in a parallel fashion by using super- or distributed- computing methods such as Folding@Home or Blue Gene. The analysis of these large, high-dimensional data sets presents new computational challenges. In this seminar, I will discuss a novel approach to analyzing large ensembles of molecular dynamics trajectories to generate a compact model of the dynamics. This model groups conformations into discrete states and describes the dynamics as Markovian, or history-independent, transitions between the states. I will discuss why the Markovian state model (MSM) is suitable for macromolecular dynamics, and how it can be used to answer many interesting and relevant questions about the molecular system. I will also discuss many of the computational and statistical challenges in building such a model, such as how to appropriately cluster conformations, determine the statistical reliability, and efficiently design new simulations.
Hokputsa, Sanya; Jumel, Kornelia; Alexander, Catherine; Harding, Stephen E
2003-08-01
Hyaluronic acid (HA) is a natural polysaccharide with importance in the pharmaceutical, medical and cosmetic industries. Determining factors in its final applications are its physicochemical properties, particularly molecular mass. A high molecular mass HA was degraded using five different hydroxyl free-radical starting concentrations chemically produced from ascorbic acid and hydrogen peroxide. The aims of the study were to investigate the effect of different hydroxyl free-radical concentrations on the chain length of HA and compare the molecular masses obtained from analytical ultracentrifugation using sedimentation equilibrium experiments and size exclusion chromatography/multi-angle laser light scattering (SEC/MALLS). The results indicated that their molecular masses varied, depending on the degree of hydroxyl free-radical starting concentration. Molecular mass values obtained from sedimentation equilibrium experiments for each sample showed the same trend as those obtained from the SEC/MALLS in the range of molecular masses studied. The molecular masses obtained from sedimentation equilibrium for high molecular mass samples from reciprocal plots of apparent weight average molecular mass against concentration gave values similar to those obtained by SEC/MALLS. In contrast, the molecular mass from conventional plots for high molecular mass samples were much lower than those from SEC/MALLS, even when high ionic strength buffers were used.
NASA Astrophysics Data System (ADS)
Yong, Xin
Nano fluidics has shown promising potential for applications that could significantly impact our daily life, such as energy harvest, lab on a chip, desalination, etc. Current techniques to realize nano fluidic ideas are still very limited due to manufacturing technology. Although sub-micron fabrication techniques are undergoing rapid development recently, scientists and engineers are still not able to access actual nanometric systems. This reason prompts the development of computational tools to reveal physical principles underlying nano fluidic phenomena. Among various numerical approaches ranging from macroscopic to microscopic, molecular dynamics stands out because of its ability to faithfully model both equilibrium and non-equilibrium nanosystems by involving an appropriate amount of molecular details. The results from molecular dynamics simulations could elucidate essential physics and benefit designs of practical nano fluidic systems. This thesis attempts to provide the theoretical foundation for modeling nano fluidic systems, by investigating nanoscale fluid behaviors and nanoscale fluid-solid interfacial physics and transport for simple fluids via molecular dynamics simulations. Boundary-driven-shear, homogeneous-shear and reverse non-equilibrium molecular dynamics methods are implemented to generate non-equilibrium systems. The fundamental fluid behaviors such as velocity profile, temperature distribution and rheological material functions under steady planar shear are explored comprehensively by each method corresponding to different perspectives. The influences of nanoscale confinement are analyzed from the comparison among these methods. The advantages and disadvantages of each method are clarified, which provide guidance to conduct appropriate molecular dynamics simulations for nano fluidics. Further studies on the intrinsic slip of smooth solid surfaces is realized by the boundary-driven-shear method. Inspired by previous hypothesis of momentum
Non-equilibrium dynamics and structure of interfacial ice
NASA Astrophysics Data System (ADS)
Andreussi, Oliviero; Donadio, Davide; Parrinello, Michele; Zewail, Ahmed H.
2006-07-01
Stimulated by recent experiments [C.-Y. Ruan et al. Science 304, (2004) 81], we have performed molecular dynamics and ab initio structural studies of the laser-induced heating and restructuring processes of nanometer-scale ice on a substrate of chlorine terminated Si(1 1 1). Starting from proton disordered cubic ice configurations the thin film behavior has been characterized at several temperatures up to the melting point. The surface induces order with crystallization in the Ic lattice, but with void amorphous regions. The structure changes on the ultrashort time scale and restructures by heat dissipation depending on the relaxation time and final temperature. Our results show the general behavior observed experimentally, thus providing the nature of forces in the atomic-scale description of interfacial ice.
Equilibrium and Dynamical Behavior of Slip in Polymer Films
NASA Astrophysics Data System (ADS)
Priezjev, Nikolai; Troian, Sandra
2003-11-01
Slip behavior in small fluidic devices can strongly modify predictions of the flow rates. In an effort to understand the fundamental basis for slip, we have conducted MD simulations of liquid films modeled as N-mer polymer chains (1≤N≤16) in steady Couette flow. The results are consistent with a slip length response of the form L_s=L^os lF(dotγ), where L^os denotes the asymptotic value of the slip length as dotγarrow 0 and lF(dotγ) is a function describing the dynamic response. We discuss the dependence of the slip length on molecular weight and show that the results for L^os agree extremely well with predictions based on a Green-Kubo analysis of the friction imposed by the wall on the first fluid layer [1]. The dynamic response lF(dotγ) is well represented by (1-dotγ/dotγ_c)-0.50 ± 0.05 even for the longest chains [2]. These findings offer an encouraging start for understanding which mechanisms contribute to large slip lengths. [1] J.L. Barrat and L. Bocquet, Phys. Rev. Lett. 82, 4671 (1999) [2] P. A. Thompson and S. M. Troian, Nature, 389, 360 (1997).
On calculating the equilibrium structure of molecular crystals.
Mattsson, Ann Elisabet; Wixom, Ryan R.; Mattsson, Thomas Kjell Rene
2010-03-01
The difficulty of calculating the ambient properties of molecular crystals, such as the explosive PETN, has long hampered much needed computational investigations of these materials. One reason for the shortcomings is that the exchange-correlation functionals available for Density Functional Theory (DFT) based calculations do not correctly describe the weak intermolecular van der Waals' forces present in molecular crystals. However, this weak interaction also poses other challenges for the computational schemes used. We will discuss these issues in the context of calculations of lattice constants and structure of PETN with a number of different functionals, and also discuss if these limitations can be circumvented for studies at non-ambient conditions.
Molecular dynamics simulation of pyridine
NASA Astrophysics Data System (ADS)
Trumpakaj, Zygmunt; Linde, Bogumił
2015-04-01
Molecular Dynamics (MD) simulations are used for the investigation of molecular motions in pyridine in the temperature range 20-480 K under normal pressure. The results obtained are analyzed within the frame of the Mori Zwanzig memory function formalism. An analytical approximation of the first memory function K(t) is applied to predict some dependences on temperature. Experimental results of the Rayleigh scattering of depolarized light from liquid pyridine are used as the main base for the comparison.
Glassy behavior of a homopolymer from molecular dynamics simulations.
Dokholyan, Nikolay V; Pitard, Estelle; Buldyrev, Sergey V; Stanley, H Eugene
2002-03-01
We study at- and out-of-equilibrium dynamics of a single homopolymer chain at low temperature using molecular dynamics. The main quantities of interest are the average root mean square displacement of the monomers below the theta point, and the structure factor, as a function of time. The observation of these quantities show a close resemblance to those measured in structural glasses and suggest that the polymer chain in its low temperature phase is in a glassy phase, with its dynamics dominated by traps. In equilibrium, at low temperature, we observe the trapping of the monomers and a slowing down of the overall motion of the polymer as well as nonexponential relaxation of the structure factor. Out of equilibrium, at low temperatures, we compute the two-time quantities and observe breaking of ergodicity in a range of waiting times, with the onset of aging.
Glassy behavior of a homopolymer from molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Dokholyan, Nikolay V.; Pitard, Estelle; Buldyrev, Sergey V.; Stanley, H. Eugene
2002-03-01
We study at- and out-of-equilibrium dynamics of a single homopolymer chain at low temperature using molecular dynamics. The main quantities of interest are the average root mean square displacement of the monomers below the θ point, and the structure factor, as a function of time. The observation of these quantities show a close resemblance to those measured in structural glasses and suggest that the polymer chain in its low temperature phase is in a glassy phase, with its dynamics dominated by traps. In equilibrium, at low temperature, we observe the trapping of the monomers and a slowing down of the overall motion of the polymer as well as nonexponential relaxation of the structure factor. Out of equilibrium, at low temperatures, we compute the two-time quantities and observe breaking of ergodicity in a range of waiting times, with the onset of aging.
Integration methods for molecular dynamics
Leimkuhler, B.J.; Reich, S.; Skeel, R.D.
1996-12-31
Classical molecular dynamics simulation of a macromolecule requires the use of an efficient time-stepping scheme that can faithfully approximate the dynamics over many thousands of timesteps. Because these problems are highly nonlinear, accurate approximation of a particular solution trajectory on meaningful time intervals is neither obtainable nor desired, but some restrictions, such as symplecticness, can be imposed on the discretization which tend to imply good long term behavior. The presence of a variety of types and strengths of interatom potentials in standard molecular models places severe restrictions on the timestep for numerical integration used in explicit integration schemes, so much recent research has concentrated on the search for alternatives that possess (1) proper dynamical properties, and (2) a relative insensitivity to the fastest components of the dynamics. We survey several recent approaches. 48 refs., 2 figs.
SEARS,T.J.; HALL,G.E.; PRESES,J.M.; WESTON,R.E.,JR.
1999-06-09
The goal of this research is the understanding of elementary chemical and physical processes important in the combustion of fossil fuels. Interest centers on reactions involving short-lived chemical intermediates and their properties. High-resolution, high-sensitivity, laser absorption methods are augmented by high temperature flow-tube reaction kinetics studies with mass-spectrometric sampling. These experiments provide information on the energy levels, structures and reactivity of molecular free radical species and, in turn, provide new tools for the study of energy flow and chemical bond cleavage in the radicals in chemical systems. The experimental work is supported by theoretical and computational work using time-dependent quantum wavepacket calculations that provide insights into energy flow between the vibrational modes of the molecule. The work of group members Fockenberg and Muckerman is described in separate abstracts of this volume.
On calculating the equilibrium structure of molecular crystals
NASA Astrophysics Data System (ADS)
Mattsson, Ann E.; Wixom, Ryan R.; Mattsson, Thomas R.
2010-03-01
The difficulty of calculating the ambient properties of molecular crystals, such as the explosive PETN, has long hampered much needed computational investigations of these materials. One reason for the shortcomings is that the exchange-correlation functionals available for Density Functional Theory (DFT) based calculations do not correctly describe the weak intermolecular van der Waals' forces present in molecular crystals. However, this weak interaction also poses other challenges for the computational schemes used. We will discuss these issues in the context of calculations of lattice constants and structure of PETN with a number of different functionals, and also discuss if these limitations can be circumvented for studies at non-ambient conditions. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
Studying non-equilibrium many-body dynamics using one-dimensional Bose gases
Langen, Tim; Gring, Michael; Kuhnert, Maximilian; Rauer, Bernhard; Geiger, Remi; Mazets, Igor; Smith, David Adu; Schmiedmayer, Jörg; Kitagawa, Takuya; Demler, Eugene
2014-12-04
Non-equilibrium dynamics of isolated quantum many-body systems play an important role in many areas of physics. However, a general answer to the question of how these systems relax is still lacking. We experimentally study the dynamics of ultracold one-dimensional (1D) Bose gases. This reveals the existence of a quasi-steady prethermalized state which differs significantly from the thermal equilibrium of the system. Our results demonstrate that the dynamics of non-equilibrium quantum many-body systems is a far richer process than has been assumed in the past.
Non equilibrium dynamics of isolated disordered systems: the classical Hamiltonian p-spin model
NASA Astrophysics Data System (ADS)
Cugliandolo, Leticia F.; Lozano, Gustavo S.; Nessi, Emilio N.
2017-08-01
We study the dynamics of a classical disordered macroscopic model completely isolated from the environment reproducing, in a classical setting, the ‘quantum quench’ protocol. We show that, depending on the pre and post quench parameters, the system approaches equilibrium, succeeding to act as a bath on itself, or remains out of equilibrium, in two different ways. In the latter one, the system stays confined in a metastable state in which it undergoes stationary dynamics characterised by a single temperature. In the other, the system ages and its dynamics are characterised by two temperatures associated with observations made at short and long time differences (high and low frequencies). The parameter dependence of the asymptotic states is rationalised in terms of a dynamic phase diagram with one equilibrium and two out of equilibrium phases. Aspects of pre-thermalisation are observed and discussed. Similarities and differences with the dynamics of the dissipative model are also explained.
Dynamic molecular graphs: "hopping" structures.
Cortés-Guzmán, Fernando; Rocha-Rinza, Tomas; Guevara-Vela, José Manuel; Cuevas, Gabriel; Gómez, Rosa María
2014-05-05
This work aims to contribute to the discussion about the suitability of bond paths and bond-critical points as indicators of chemical bonding defined within the theoretical framework of the quantum theory of atoms in molecules. For this purpose, we consider the temporal evolution of the molecular structure of [Fe{C(CH2 )3 }(CO)3 ] throughout Born-Oppenheimer molecular dynamics (BOMD), which illustrates the changing behaviour of the molecular graph (MG) of an electronic system. Several MGs with significant lifespans are observed across the BOMD simulations. The bond paths between the trimethylenemethane and the metallic core are uninterruptedly formed and broken. This situation is reminiscent of a "hopping" ligand over the iron atom. The molecular graph wherein the bonding between trimethylenemethane and the iron atom takes place only by means of the tertiary carbon atom has the longest lifespan of all the considered structures, which is consistent with the MG found by X-ray diffraction experiments and quantum chemical calculations. In contrast, the η(4) complex predicted by molecular-orbital theory has an extremely brief lifetime. The lifespan of different molecular structures is related to bond descriptors on the basis of the topology of the electron density such as the ellipticities at the FeCH2 bond-critical points and electron delocalisation indices. This work also proposes the concept of a dynamic molecular graph composed of the different structures found throughout the BOMD trajectories in analogy to a resonance hybrid of Lewis structures. It is our hope that the notion of dynamic molecular graphs will prove useful in the discussion of electronic systems, in particular for those in which analysis on the basis of static structures leads to controversial conclusions.
Out-of-equilibrium competitive dynamics of quasispecies
NASA Astrophysics Data System (ADS)
Aguirre, J.; Manrubia, S. C.
2007-02-01
The composition of a quasispecies is completely characterized, in the large population and long time limit, by the matrix yielding the transition probabilities between different types in the population. Further, its asymptotic growth rate —i.e. the largest eigenvalue of the transition matrix— completely determines the winning population in an equilibrium competition. However, due to the intrinsically heterogeneous nature of quasispecies, out-of-equilibrium fluctuations in population size might change the expected fate of competition experiments. Using a simple model for a heterogeneous population we quantify the probability that, after a population bottleneck, the a priori weaker competitor wins in a competition with a population characterized by a larger asymptotic growth rate. We analyse the role played by different degrees of neutrality in the outcome of the process, and demonstrate that lower neutrality favours the weaker competitor in out-of-equilibrium situations. Our results might shed light on empirical observations in competition experiments with RNA viruses.
A quantum molecular dynamics study of aqueous solvation dynamics
NASA Astrophysics Data System (ADS)
Videla, Pablo E.; Rossky, Peter J.; Laria, D.
2013-10-01
Ring polymer molecular dynamics experiments have been carried out to examine effects derived from nuclear quantum fluctuations at ambient conditions on equilibrium and non-equilibrium dynamical characteristics of charge solvation by a popular simple, rigid, water model, SPC/E, and for a more recent, and flexible, q-TIP4P/F model, to examine the generality of conclusions. In particular, we have recorded the relaxation of the solvent energy gap following instantaneous, ±e charge jumps in an initially uncharged Lennard-Jones-like solute. In both charge cases, quantum effects are reflected in sharper decays at the initial stages of the relaxation, which produce up to a ˜20% reduction in the characteristic timescales describing the solvation processes. For anionic solvation, the magnitude of polarization fluctuations controlling the extent of the water proton localization in the first solvation shell is somewhat more marked than for cations, bringing the quantum solvation process closer to the classical case. Effects on the solvation response from the explicit incorporation of flexibility in the water Hamiltonian are also examined. Predictions from linear response theories for the overall relaxation profile and for the corresponding characteristic timescales are reasonably accurate for the solvation of cations, whereas we find that they are much less satisfactory for the anionic case.
Collisional dynamics in a gas of molecular super-rotors.
Khodorkovsky, Yuri; Steinitz, Uri; Hartmann, Jean-Michel; Averbukh, Ilya Sh
2015-07-10
Recently, femtosecond laser techniques have been developed that are capable of bringing gas molecules to extremely fast rotation in a very short time, while keeping their translational motion relatively slow. Here we study collisional equilibration dynamics of this new state of molecular gases. We show that the route to equilibrium starts with a metastable 'gyroscopic stage' in the course of which the molecules maintain their fast rotation and orientation of the angular momentum through many collisions. The inhibited rotational-translational relaxation is characterized by a persistent anisotropy in the molecular angular distribution, and is manifested in the optical birefringence and anisotropic diffusion in the gas. After a certain induction time, the 'gyroscopic stage' is abruptly terminated by an explosive rotational-translational energy exchange, leading the gas towards the final equilibrium. We illustrate our conclusions by direct molecular dynamics simulation of several gases of linear molecules.
Equilibrium analysis of the efficiency of an autonomous molecular computer
NASA Astrophysics Data System (ADS)
Rose, John A.; Deaton, Russell J.; Hagiya, Masami; Suyama, Akira
2002-02-01
In the whiplash polymerase chain reaction (WPCR), autonomous molecular computation is implemented in vitro by the recursive, self-directed polymerase extension of a mixture of DNA hairpins. Although computational efficiency is known to be reduced by a tendency for DNAs to self-inhibit by backhybridization, both the magnitude of this effect and its dependence on the reaction conditions have remained open questions. In this paper, the impact of backhybridization on WPCR efficiency is addressed by modeling the recursive extension of each strand as a Markov chain. The extension efficiency per effective polymerase-DNA encounter is then estimated within the framework of a statistical thermodynamic model. Model predictions are shown to provide close agreement with the premature halting of computation reported in a recent in vitro WPCR implementation, a particularly significant result, given that backhybridization had been discounted as the dominant error process. The scaling behavior further indicates completion times to be sufficiently long to render WPCR-based massive parallelism infeasible. A modified architecture, PNA-mediated WPCR (PWPCR) is then proposed in which the occupancy of backhybridized hairpins is reduced by targeted PNA2/DNA triplex formation. The efficiency of PWPCR is discussed using a modified form of the model developed for WPCR. Predictions indicate the PWPCR efficiency is sufficient to allow the implementation of autonomous molecular computation on a massive scale.
ERIC Educational Resources Information Center
Chiu, Mei-Hung; Chou, Chin-Cheng; Liu, Chia-Ju
2002-01-01
Investigates students' mental models of chemical equilibrium using dynamic science assessments. Reports that students at various levels have misconceptions about chemical equilibrium. Involves 10th grade students (n=30) in the study doing a series of hands-on chemical experiments. Focuses on the process of constructing mental models, dynamic…
ERIC Educational Resources Information Center
Chiu, Mei-Hung; Chou, Chin-Cheng; Liu, Chia-Ju
2002-01-01
Investigates students' mental models of chemical equilibrium using dynamic science assessments. Reports that students at various levels have misconceptions about chemical equilibrium. Involves 10th grade students (n=30) in the study doing a series of hands-on chemical experiments. Focuses on the process of constructing mental models, dynamic…
Molecular dynamics in high electric fields
NASA Astrophysics Data System (ADS)
Apostol, M.; Cune, L. C.
2016-06-01
Molecular rotation spectra, generated by the coupling of the molecular electric-dipole moments to an external time-dependent electric field, are discussed in a few particular conditions which can be of some experimental interest. First, the spherical-pendulum molecular model is reviewed, with the aim of introducing an approximate method which consists in the separation of the azimuthal and zenithal motions. Second, rotation spectra are considered in the presence of a static electric field. Two particular cases are analyzed, corresponding to strong and weak fields. In both cases the classical motion of the dipoles consists of rotations and vibrations about equilibrium positions; this motion may exhibit parametric resonances. For strong fields a large macroscopic electric polarization may appear. This situation may be relevant for polar matter (like pyroelectrics, ferroelectrics), or for heavy impurities embedded in a polar solid. The dipolar interaction is analyzed in polar condensed matter, where it is shown that new polarization modes appear for a spontaneous macroscopic electric polarization (these modes are tentatively called "dipolons"); one of the polarization modes is related to parametric resonances. The extension of these considerations to magnetic dipoles is briefly discussed. The treatment is extended to strong electric fields which oscillate with a high frequency, as those provided by high-power lasers. It is shown that the effect of such fields on molecular dynamics is governed by a much weaker, effective, renormalized, static electric field.
Effective interactions in molecular dynamics simulations of lysozyme solutions
NASA Astrophysics Data System (ADS)
Pellicane, Giuseppe; Sarkisov, Lev
2014-09-01
In this article we explore a problem of effective interactions between two rotationally restrained lysozyme molecules forming a crystal contact in aqueous solution. We perform non-equilibrium molecular dynamics simulations in order to estimate the interaction energy as a function of the distance between the two proteins obtained from direct application of the Jarzynski equality (JE), and compare it with that calculated by means of another non-equilibrium approach (Forward-Reverse method) and constrained force methods. The performance of the JE equality when applied to solvated protein interactions is discussed. All of the equilibrium and non-equilibrium methods show clear evidence that the potentials of mean force (PMF) are short-ranged, do not exceed few kTs, and that there is an accumulation of anions in the presence of hydrophobic surfaces.
Nanodrop contact angles from molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Ravipati, Srikanth; Aymard, Benjamin; Yatsyshin, Petr; Galindo, Amparo; Kalliadasis, Serafim
2016-11-01
The contact angle between three phases being in thermodynamic equilibrium is highly sensitive to the nature of the intermolecular forces as well as to various fluctuation effects. Determining the Young contact angle of a sessile drop sitting on a substrate from molecular dynamics (MD) simulations is a highly non-trivial task. Most commonly employed methods for finding droplet contact angles from MD simulation data either require large numbers of particles or are system-dependent. We propose a systematic geometry based methodology for extracting the contact angle from simulated sessile droplets by analysing an appropriately coarse-grained density field. To demonstrate the method, we consider Lennard-Jones (LJ) and SPC/E water nanodroplets of different sizes sitting on planar LJ walls. Our results are in good agreement with Young contact angle values computed employing test-area perturbation method.
Nonequilibrium molecular dynamics: The first 25 years
Hoover, W.G. |
1992-08-01
Equilibrium Molecular Dynamics has been generalized to simulate Nonequilibrium systems by adding sources of thermodynamic heat and work. This generalization incorporates microscopic mechanical definitions of macroscopic thermodynamic and hydrodynamic variables, such as temperature and stress, and augments atomistic forces with special boundary, constraint, and driving forces capable of doing work on, and exchanging heat with, an otherwise Newtonian system. The underlying Lyapunov instability of these nonequilibrium equations of motion links microscopic time-reversible deterministic trajectories to macroscopic time-irreversible hydrodynamic behavior as described by the Second Law of Thermodynamics. Green-Kubo linear-response theory has been checked. Nonlinear plastic deformation, intense heat conduction, shockwave propagation, and nonequilibrium phase transformation have all been simulated. The nonequilibrium techniques, coupled with qualitative improvements in parallel computer hardware, are enabling simulations to approximate real-world microscale and nanoscale experiments.
Cell list algorithms for nonequilibrium molecular dynamics
NASA Astrophysics Data System (ADS)
Dobson, Matthew; Fox, Ian; Saracino, Alexandra
2016-06-01
We present two modifications of the standard cell list algorithm that handle molecular dynamics simulations with deforming periodic geometry. Such geometry naturally arises in the simulation of homogeneous, linear nonequilibrium flow modeled with periodic boundary conditions, and recent progress has been made developing boundary conditions suitable for general 3D flows of this type. Previous works focused on the planar flows handled by Lees-Edwards or Kraynik-Reinelt boundary conditions, while the new versions of the cell list algorithm presented here are formulated to handle the general 3D deforming simulation geometry. As in the case of equilibrium, for short-ranged pairwise interactions, the cell list algorithm reduces the computational complexity of the force computation from O(N2) to O(N), where N is the total number of particles in the simulation box. We include a comparison of the complexity and efficiency of the two proposed modifications of the standard algorithm.
The anharmonic force field and equilibrium molecular structure of ketene
NASA Astrophysics Data System (ADS)
East, Allan L. L.; Allen, Wesley D.; Klippenstein, Stephen J.
1995-06-01
be problematic. From high-precision empirical rotational constants of six isotopomers and the theoretical anharmonic force field, the equilibrium structure of ketene is derived: re(C=O)=1.160 30(29) Å, re(C=C)=1.312 12(30) Å, re(C-H)=1.075 76(7) Å, and θe(H-C-H)=121.781(12)°. A natural bond orbital (NBO) analysis shows that the unusually large methylene angle is attributable to extensive in-plane π delocalization.
Oleuropein: Molecular Dynamics and Computation.
Gentile, Luigi; Uccella, Nicola A; Sivakumar, Ganapathy
2017-09-11
Olive oil and table olive biophenols have been shown to significantly enrich the hedonic-sensory and nutritional quality of the Mediterranean diet. Oleuropein is one of the predominate biophenols in green olives and leaves, which not only has noteworthy free-radical quenching activity but also putatively reduces the incidence of various cancers. Clinical trials suggest that the consumption of extra virgin olive oil reduces the risk of several degenerative diseases. The oleuropein-based bioactives in olive oil could reduce tumor necrosis factor α, interleukin-1β and nitric oxide. Therefore, olive bioactives quality should be preserved and even improved due to their disease-fighting properties. Understanding the molecular dynamics of oleuropein is crucial to increase olive oil and table olive quality. The objective of this review is to provide the molecular dynamics and computational mapping of oleuropein. It is a biophenol-secoiridoid expressing different functionalities such as two π-bonds, two esters, two acetals, one catechol, and four hexose hydroxyls within 540 mw. The molecular bond sequential breaking mechanisms were analyzed through unimolecular reactions under electron spray ionization, collision activated dissociations, and fast atom bombardment mass spectrometry. The oleuropein solvent-free reactivity is leading to glucose loss and bioactive aglycone-dialdehydes via secoiridoid ring opening. Oleuropein electron distribution revealed that the free-radical non-polar processes occur from its highest occupied molecular orbital, while the lowest unoccupied molecular orbital is clearly devoted to nucleophilic and base site reactivity. This molecular dynamics and computational mapping of oleuropein could contribute to the engineering of olive-based biomedicine and/or functional food. Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.org.
Tassis, Konstantinos; Willacy, Karen; Yorke, Harold W.; Turner, Neal J.
2012-07-20
We study the effect that non-equilibrium chemistry in dynamical models of collapsing molecular cloud cores has on measurements of the magnetic field in these cores, the degree of ionization, and the mean molecular weight of ions. We find that OH and CN, usually used in Zeeman observations of the line-of-sight magnetic field, have an abundance that decreases toward the center of the core much faster than the density increases. As a result, Zeeman observations tend to sample the outer layers of the core and consistently underestimate the core magnetic field. The degree of ionization follows a complicated dependence on the number density at central densities up to 10{sup 5} cm{sup -3} for magnetic models and 10{sup 6} cm{sup -3} in non-magnetic models. At higher central densities, the scaling approaches a power law with a slope of -0.6 and a normalization which depends on the cosmic-ray ionization rate {zeta} and the temperature T as ({zeta}T){sup 1/2}. The mean molecular weight of ions is systematically lower than the usually assumed value of 20-30, and, at high densities, approaches a value of 3 due to the asymptotic dominance of the H{sup +}{sub 3} ion. This significantly lower value implies that ambipolar diffusion operates faster.
Out-of-equilibrium dynamics of gene expression and the Jarzynski equality.
Berg, Johannes
2008-05-09
In order to express specific genes at the right time, the transcription of genes is regulated by the presence and absence of transcription factor molecules. With transcription factor concentrations undergoing constant changes, gene transcription takes place out of equilibrium. In this Letter we discuss a simple mapping between dynamic models of gene expression and stochastic systems driven out of equilibrium. Using this mapping, results of out-of-equilibrium statistical mechanics such as the Jarzynski equality and the fluctuation theorem are demonstrated for gene expression dynamics. Applications of this approach include the determination of regulatory interactions between genes from experimental gene expression data.
Available Instruments for Analyzing Molecular Dynamics Trajectories
Likhachev, I. V.; Balabaev, N. K.; Galzitskaya, O. V.
2016-01-01
Molecular dynamics trajectories are the result of molecular dynamics simulations. Trajectories are sequential snapshots of simulated molecular system which represents atomic coordinates at specific time periods. Based on the definition, in a text format trajectory files are characterized by their simplicity and uselessness. To obtain information from such files, special programs and information processing techniques are applied: from molecular dynamics animation to finding characteristics along the trajectory (versus time). In this review, we describe different programs for processing molecular dynamics trajectories. The performance of these programs, usefulness for analyses of molecular dynamics trajectories, strong and weak aspects are discussed. PMID:27053964
Las Palmeras Molecular Dynamics: A flexible and modular molecular dynamics code
NASA Astrophysics Data System (ADS)
Davis, Sergio; Loyola, Claudia; González, Felipe; Peralta, Joaquín
2010-12-01
Las Palmeras Molecular Dynamics (LPMD) is a highly modular and extensible molecular dynamics (MD) code using interatomic potential functions. LPMD is able to perform equilibrium MD simulations of bulk crystalline solids, amorphous solids and liquids, as well as non-equilibrium MD (NEMD) simulations such as shock wave propagation, projectile impacts, cluster collisions, shearing, deformation under load, heat conduction, heterogeneous melting, among others, which involve unusual MD features like non-moving atoms and walls, unstoppable atoms with constant-velocity, and external forces like electric fields. LPMD is written in C++ as a compromise between efficiency and clarity of design, and its architecture is based on separate components or plug-ins, implemented as modules which are loaded on demand at runtime. The advantage of this architecture is the ability to completely link together the desired components involved in the simulation in different ways at runtime, using a user-friendly control file language which describes the simulation work-flow. As an added bonus, the plug-in API (Application Programming Interface) makes it possible to use the LPMD components to analyze data coming from other simulation packages, convert between input file formats, apply different transformations to saved MD atomic trajectories, and visualize dynamical processes either in real-time or as a post-processing step. Individual components, such as a new potential function, a new integrator, a new file format, new properties to calculate, new real-time visualizers, and even a new algorithm for handling neighbor lists can be easily coded, compiled and tested within LPMD by virtue of its object-oriented API, without the need to modify the rest of the code. LPMD includes already several pair potential functions such as Lennard-Jones, Morse, Buckingham, MCY and the harmonic potential, as well as embedded-atom model (EAM) functions such as the Sutton-Chen and Gupta potentials. Integrators to
Novel methods for molecular dynamics simulations.
Elber, R
1996-04-01
In the past year, significant progress was made in the development of molecular dynamics methods for the liquid phase and for biological macromolecules. Specifically, faster algorithms to pursue molecular dynamics simulations were introduced and advances were made in the design of new optimization algorithms guided by molecular dynamics protocols. A technique to calculate the quantum spectra of protein vibrations was introduced.
Equilibrium and non-equilibrium dynamics of the dilute lamellar phase
NASA Astrophysics Data System (ADS)
Ramaswamy, Sriram
1992-07-01
A model for the dynamics of the sterically stabilized dilute lamellar phase is constructed and studied. The model consists of a stack of flexible fluid sheets, with excluded volume, separated by macroscopic layers of solvent. The dynamics of small fluctuations of the sheets about their mean positions is found to have two distinct short-wavelength regimes in which the frequency ω depends on the wavenumber q in an unusual manner. One is a single-membrane Zimm mode, ω ≈ - iq3, while the other is a “red-blood-cell mode”, ω ≈ - iq6. These modes give rise to fluctuation corrections for the viscosities of the system, going as ω {-1}/{3} and ω {-2}/{3}, respectively. In addition, it is shown that a sufficiently rapid shear flow with velocity and gradient in the plane of the layers causes a transition into a state where regions of reduced layer spacing co-exist with regions devoid of any layer material. The critical shear-rate for this transition should go as (layer spacing) -3. Possible experimental tests of these predictions are discussed.
Equilibrium, dynamic, and trapping properties of an excess electron in dense helium
NASA Astrophysics Data System (ADS)
Sheu, Sheh-Yi; Cukier, R. I.
1991-06-01
The equilibrium, dynamic, and trapping properties of an excess electron in dense helium are simulated. An adiabatic simulation method is used whereby the Schrödinger equation for the electron in the presence of a fixed, classical solvent configuration is solved. The solvent configuration is advanced by molecular dynamics with the force on a particular helium atom arising from the classical helium-helium potential and the expectation value of the electron-helium potential. The equilibrium properties of the electron are contrasted with those obtained by Coker and Berne [D. F. Coker and B. F. Berne, J. Chem. Phys. 89, 2128 (1988)] using a different procedure for generating helium configurations. The diffusion coefficient of the electron is obtained and, for ρ*=ρσ 3=0.9, is De=5.0×10-3 cm2 s-1. This is an order of magnitude greater than the diffusion coefficient of the helium atoms and corresponds to a very mobile electron. The distribution of times for an electron to move between donor and acceptor sites inserted in the system is obtained and shown to yield an average diffusion coefficient consistent with that obtained from the mean square displacement. The ability of the electron to move between the donor and acceptor sites by electron transfer is assessed by evaluating the reorganization energy of the solvent and using conventional electron transfer theory. If the sites are sufficiently far apart, then electron transport via detrapping from the donor site followed by transport to the acceptor site can be competitive with electron transfer as a charge transport mechanism.
Molecular dynamics modeling of a nanomaterials-water surface interaction
NASA Astrophysics Data System (ADS)
Nejat Pishkenari, Hossein; Keramati, Ramtin; Abdi, Ahmad; Minary-Jolandan, Majid
2016-04-01
In this article, we study the formation of nanomeniscus around a nanoneedle using molecular dynamics simulation approach. The results reveal three distinct phases in the time-evolution of meniscus before equilibrium according to the contact angle, meniscus height, and potential energy. In addition, we investigated the correlation between the nanoneedle diameter and nanomeniscus characteristics. The results have applications in various fields such as scanning probe microscopy and rheological measurements.
Scalable Molecular Dynamics with NAMD
Phillips, James C.; Braun, Rosemary; Wang, Wei; Gumbart, James; Tajkhorshid, Emad; Villa, Elizabeth; Chipot, Christophe; Skeel, Robert D.; Kalé, Laxmikant; Schulten, Klaus
2008-01-01
NAMD is a parallel molecular dynamics code designed for high-performance simulation of large biomolecular systems. NAMD scales to hundreds of processors on high-end parallel platforms, as well as tens of processors on low-cost commodity clusters, and also runs on individual desktop and laptop computers. NAMD works with AMBER and CHARMM potential functions, parameters, and file formats. This paper, directed to novices as well as experts, first introduces concepts and methods used in the NAMD program, describing the classical molecular dynamics force field, equations of motion, and integration methods along with the efficient electrostatics evaluation algorithms employed and temperature and pressure controls used. Features for steering the simulation across barriers and for calculating both alchemical and conformational free energy differences are presented. The motivations for and a roadmap to the internal design of NAMD, implemented in C++ and based on Charm++ parallel objects, are outlined. The factors affecting the serial and parallel performance of a simulation are discussed. Next, typical NAMD use is illustrated with representative applications to a small, a medium, and a large biomolecular system, highlighting particular features of NAMD, e.g., the Tcl scripting language. Finally, the paper provides a list of the key features of NAMD and discusses the benefits of combining NAMD with the molecular graphics/sequence analysis software VMD and the grid computing/collaboratory software BioCoRE. NAMD is distributed free of charge with source code at www.ks.uiuc.edu. PMID:16222654
Scalable molecular dynamics with NAMD.
Phillips, James C; Braun, Rosemary; Wang, Wei; Gumbart, James; Tajkhorshid, Emad; Villa, Elizabeth; Chipot, Christophe; Skeel, Robert D; Kalé, Laxmikant; Schulten, Klaus
2005-12-01
NAMD is a parallel molecular dynamics code designed for high-performance simulation of large biomolecular systems. NAMD scales to hundreds of processors on high-end parallel platforms, as well as tens of processors on low-cost commodity clusters, and also runs on individual desktop and laptop computers. NAMD works with AMBER and CHARMM potential functions, parameters, and file formats. This article, directed to novices as well as experts, first introduces concepts and methods used in the NAMD program, describing the classical molecular dynamics force field, equations of motion, and integration methods along with the efficient electrostatics evaluation algorithms employed and temperature and pressure controls used. Features for steering the simulation across barriers and for calculating both alchemical and conformational free energy differences are presented. The motivations for and a roadmap to the internal design of NAMD, implemented in C++ and based on Charm++ parallel objects, are outlined. The factors affecting the serial and parallel performance of a simulation are discussed. Finally, typical NAMD use is illustrated with representative applications to a small, a medium, and a large biomolecular system, highlighting particular features of NAMD, for example, the Tcl scripting language. The article also provides a list of the key features of NAMD and discusses the benefits of combining NAMD with the molecular graphics/sequence analysis software VMD and the grid computing/collaboratory software BioCoRE. NAMD is distributed free of charge with source code at www.ks.uiuc.edu. (c) 2005 Wiley Periodicals, Inc.
Large Time Out-of-Equilibrium Dynamics of a Manifold in a Random Potential
NASA Astrophysics Data System (ADS)
Cugliandolo, Leticia F.; Kurchan, Jorge; Le Doussal, Pierre
1996-03-01
We study the out-of-equilibrium dynamics of an elastic manifold in a random potential using mean-field theory. We find two asymptotic time regimes: (i) stationary dynamics; (ii) slow aging dynamics with violation of equilibrium theorems. We obtain an analytical solution valid for all large times with universal scalings of two-time quantities with space. A nonanalytic scaling function crosses over to ultrametricity when the correlations become long range. We propose procedures to test numerically or experimentally the extent to which this scenario holds for a given system.
Dynamical Systems Based Non Equilibrium Statistical Mechanics for Markov Chains
NASA Astrophysics Data System (ADS)
Prevost, Mireille
We introduce an abstract framework concerning non-equilibrium statistical mechanics in the specific context of Markov chains. This framework encompasses both the Evans-Searles and the Gallavotti-Cohen fluctuation theorems. To support and expand on these concepts, several results are proven, among which a central limit theorem and a large deviation principle. The interest for Markov chains is twofold. First, they model a great variety of physical systems. Secondly, their simplicity allows for an easy introduction to an otherwise complicated field encompassing the statistical mechanics of Anosov and Axiom A diffeomorphisms. We give two examples relating the present framework to physical cases modelled by Markov chains. One of these concerns chemical reactions and links key concepts from the framework to their well known physical counterpart.
Predicting out-of-Equilibrium Phase Behavior in the Dynamic Self-Assembly of Colloidal Crystals
NASA Astrophysics Data System (ADS)
Swan, James; Sherman, Zachary
Crystals self-assembled from colloidal particles are useful in an array of well demonstrated applications. During fabrication however, gelation and glassification often leave these materials arrested in defective or disordered metastable states. We show how time-dependent, pulsed interparticle interactions can avoid kinetic barriers and yield well-ordered crystalline domains for a suspension of hard, spherical colloidal particles interacting through short-range attractions. This dynamic self-assembly process is analogous to the flashing Brownian rachet. Although this is an inherently unsteady, out-of-equilibrium process, we can predict its outcome using appropriate time averages of equilibrium equations of state. The predicted phase behavior is tested and validated by examining the fluid/crystal coexistence of such dynamically self-assembling dispersions in Brownian dynamics simulations of sedimentation equilibrium and homogeneous nucleation. We also show that our dynamic self-assembly scheme offers control and tunability over the crystal growth kinetics and can even stabilize nonequilibrium structures.
Non-equilibrium dynamics of glass-forming liquid mixtures.
Sánchez-Díaz, Luis Enrique; Lázaro-Lázaro, Edilio; Olais-Govea, José Manuel; Medina-Noyola, Magdaleno
2014-06-21
The non-equilibrium self-consistent generalized Langevin equation theory of irreversible processes in glass-forming liquids [P. Ramírez-González and M. Medina-Noyola, Phys. Rev. E 82, 061503 (2010)] is extended here to multi-component systems. The resulting theory describes the statistical properties of the instantaneous local particle concentration profiles nα(r, t) of species α in terms of the coupled time-evolution equations for the mean value n̄α(r, t) and for the covariance σ(αβ)(r, r'; t) ≡ δn(α)(r, t)δn(β)(r', t) of the fluctuations δn(α)(r, t) = n(α)(r, t) - n̄α(r, t). As in the monocomponent case, these two coarse-grained equations involve a local mobility function bα(r, t) for each species, written in terms of the memory function of the two-time correlation function C(αβ)(r, r'; t, t') ≡ δn(α)(r, t)δn(β)(r', t'). If the system is constrained to remain spatially uniform and subjected to a non-equilibrium preparation protocol described by a given temperature and composition change program T(t) and n̄α(r, t), these equations predict the irreversible structural relaxation of the partial static structure factors Sαβ(k; t) and of the (collective and self) intermediate scattering functions Fαβ(k, τ; t) and F(αβ)(S)(k, τ; t). We illustrate the applicability of the resulting theory with two examples involving simple model mixtures subjected to an instantaneous temperature quench: an electroneutral binary mixture of equally sized and oppositely charged hard-spheres, and a binary mixture of soft-spheres of moderate size-asymmetry.
Non-equilibrium dynamics of glass-forming liquid mixtures
NASA Astrophysics Data System (ADS)
Sánchez-Díaz, Luis Enrique; Lázaro-Lázaro, Edilio; Olais-Govea, José Manuel; Medina-Noyola, Magdaleno
2014-06-01
The non-equilibrium self-consistent generalized Langevin equation theory of irreversible processes in glass-forming liquids [P. Ramírez-González and M. Medina-Noyola, Phys. Rev. E 82, 061503 (2010)] is extended here to multi-component systems. The resulting theory describes the statistical properties of the instantaneous local particle concentration profiles nα(r, t) of species α in terms of the coupled time-evolution equations for the mean value overline{n}_α ({r},t) and for the covariance σ _{α β }({r},{r}^' };t)equiv overline{δ n_α ({r},t)δ n_β ({r}^' },t)} of the fluctuations δ n_α ({r},t) = n_α ({r},t)- overline{n}_α ({r},t). As in the monocomponent case, these two coarse-grained equations involve a local mobility function bα(r, t) for each species, written in terms of the memory function of the two-time correlation function C_{α β }({r},{r}^' };t,t^' }) equiv overline{δ n_α ({r},t)δ n_β ({r}^' },t^' })}. If the system is constrained to remain spatially uniform and subjected to a non-equilibrium preparation protocol described by a given temperature and composition change program T(t) and overline{n}_α (t), these equations predict the irreversible structural relaxation of the partial static structure factors Sαβ(k; t) and of the (collective and self) intermediate scattering functions Fαβ(k, τ; t) and F^S_{α β }(k,τ ;t). We illustrate the applicability of the resulting theory with two examples involving simple model mixtures subjected to an instantaneous temperature quench: an electroneutral binary mixture of equally sized and oppositely charged hard-spheres, and a binary mixture of soft-spheres of moderate size-asymmetry.
Jing, Yuanyuan; Chen, Liping; Bai, Shuming; Shi, Qiang
2013-01-28
The hierarchical equations of motion (HEOM) method was applied to calculate the emission spectra of molecular aggregates using the Frenkel exciton model. HEOM equations for the one-exciton excited state were first propagated until equilibration. The reduced density operator and auxiliary density operators (ADOs) were used to characterize the coupled system-bath equilibrium. The dipole-dipole correlation functions were then calculated to obtain the emission spectra of model dimers, and the B850 band of light-harvesting complex II (LH2) in purple bacteria. The effect of static disorder on equilibrium excited state and the emission spectra of LH2 was also explicitly considered. Several approximation schemes, including the high temperature approximation (HTA) of the HEOM, a modified version of the HTA, the stochastic Liouville equation approach, the perturbative time-local and time-nonlocal generalized quantum master equations, were assessed in the calculation of the equilibrium excited state and emission spectra.
NASA Astrophysics Data System (ADS)
Mazilu, Irina; Gonzalez, Joshua
2008-03-01
From the point of view of a physicist, a bio-molecular motor represents an interesting non-equilibrium system and it is directly amenable to an analysis using standard methods of non-equilibrium statistical physics. We conduct a rigorous Monte Carlo study of three different driven lattice gas models that retain the basic behavior of three types of cytoskeletal molecular motors. Our models incorporate novel features such as realistic dynamics rules and complex motor-motor interactions. We are interested to have a deeper understanding of how various parameters influence the macroscopic behavior of these systems, what is the density profile and if the system undergoes a phase transition. On the analytical front, we computed the steady-state probability distributions exactly for the one of the models using the matrix method that was established in 1993 by B. Derrida et al. We also explored the possibilities offered by the ``Bethe ansatz'' method by mapping some well studied spin models into asymmetric simple exclusion models (already analyzed using computer simulations), and to use the results obtained for the spin models in finding an exact solution for our problem. We have exhaustive computational studies of the kinesin and dynein molecular motor models that prove to be very useful in checking our analytical work.
Imaging the equilibrium state and magnetization dynamics of partially built hard disk write heads
Valkass, R. A. J. Yu, W.; Shelford, L. R.; Keatley, P. S.; Loughran, T. H. J.; Hicken, R. J.; Cavill, S. A.; Laan, G. van der; Dhesi, S. S.; Bashir, M. A.; Gubbins, M. A.; Czoschke, P. J.; Lopusnik, R.
2015-06-08
Four different designs of partially built hard disk write heads with a yoke comprising four repeats of NiFe (1 nm)/CoFe (50 nm) were studied by both x-ray photoemission electron microscopy (XPEEM) and time-resolved scanning Kerr microscopy (TRSKM). These techniques were used to investigate the static equilibrium domain configuration and the magnetodynamic response across the entire structure, respectively. Simulations and previous TRSKM studies have made proposals for the equilibrium domain configuration of similar structures, but no direct observation of the equilibrium state of the writers has yet been made. In this study, static XPEEM images of the equilibrium state of writer structures were acquired using x-ray magnetic circular dichroism as the contrast mechanism. These images suggest that the crystalline anisotropy dominates the equilibrium state domain configuration, but competition with shape anisotropy ultimately determines the stability of the equilibrium state. Dynamic TRSKM images were acquired from nominally identical devices. These images suggest that a longer confluence region may hinder flux conduction from the yoke into the pole tip: the shorter confluence region exhibits clear flux beaming along the symmetry axis, whereas the longer confluence region causes flux to conduct along one edge of the writer. The observed variations in dynamic response agree well with the differences in the equilibrium magnetization configuration visible in the XPEEM images, confirming that minor variations in the geometric design of the writer structure can have significant effects on the process of flux beaming.
Near-equilibrium polymorphic phase transformations in Praseodymium under dynamic compression
Bastea, M; Reisman, D
2007-02-12
We report the first experimental observation of sequential, multiple polymorphic phase transformations occurring in Praseodymium dynamically compressed using a ramp wave. The experiments also display the signatures of reverse transformations occuring upon pressure release and reveal the presence of small hysteresys loops. The results are in very good agreement with equilibrium hydrodynamic calculations performed using a thermodynamically consistent, multi-phase equation of state for Praseodymium, suggesting a near-equilibrium transformation behavior.
Better, Cheaper, Faster Molecular Dynamics
NASA Technical Reports Server (NTRS)
Pohorille, Andrew; DeVincenzi, Donald L. (Technical Monitor)
2001-01-01
Recent, revolutionary progress in genomics and structural, molecular and cellular biology has created new opportunities for molecular-level computer simulations of biological systems by providing vast amounts of data that require interpretation. These opportunities are further enhanced by the increasing availability of massively parallel computers. For many problems, the method of choice is classical molecular dynamics (iterative solving of Newton's equations of motion). It focuses on two main objectives. One is to calculate the relative stability of different states of the system. A typical problem that has' such an objective is computer-aided drug design. Another common objective is to describe evolution of the system towards a low energy (possibly the global minimum energy), "native" state. Perhaps the best example of such a problem is protein folding. Both types of problems share the same difficulty. Often, different states of the system are separated by high energy barriers, which implies that transitions between these states are rare events. This, in turn, can greatly impede exploration of phase space. In some instances this can lead to "quasi non-ergodicity", whereby a part of phase space is inaccessible on time scales of the simulation. To overcome this difficulty and to extend molecular dynamics to "biological" time scales (millisecond or longer) new physical formulations and new algorithmic developments are required. To be efficient they should account for natural limitations of multi-processor computer architecture. I will present work along these lines done in my group. In particular, I will focus on a new approach to calculating the free energies (stability) of different states and to overcoming "the curse of rare events". I will also discuss algorithmic improvements to multiple time step methods and to the treatment of slowly decaying, log-ranged, electrostatic effects.
Better, Cheaper, Faster Molecular Dynamics
NASA Technical Reports Server (NTRS)
Pohorille, Andrew; DeVincenzi, Donald L. (Technical Monitor)
2001-01-01
Recent, revolutionary progress in genomics and structural, molecular and cellular biology has created new opportunities for molecular-level computer simulations of biological systems by providing vast amounts of data that require interpretation. These opportunities are further enhanced by the increasing availability of massively parallel computers. For many problems, the method of choice is classical molecular dynamics (iterative solving of Newton's equations of motion). It focuses on two main objectives. One is to calculate the relative stability of different states of the system. A typical problem that has' such an objective is computer-aided drug design. Another common objective is to describe evolution of the system towards a low energy (possibly the global minimum energy), "native" state. Perhaps the best example of such a problem is protein folding. Both types of problems share the same difficulty. Often, different states of the system are separated by high energy barriers, which implies that transitions between these states are rare events. This, in turn, can greatly impede exploration of phase space. In some instances this can lead to "quasi non-ergodicity", whereby a part of phase space is inaccessible on time scales of the simulation. To overcome this difficulty and to extend molecular dynamics to "biological" time scales (millisecond or longer) new physical formulations and new algorithmic developments are required. To be efficient they should account for natural limitations of multi-processor computer architecture. I will present work along these lines done in my group. In particular, I will focus on a new approach to calculating the free energies (stability) of different states and to overcoming "the curse of rare events". I will also discuss algorithmic improvements to multiple time step methods and to the treatment of slowly decaying, log-ranged, electrostatic effects.
Molecular dynamics analysis of a liquid explosive reaction zone
NASA Astrophysics Data System (ADS)
Soulard, Laurent
2005-07-01
We present in this work a detailed analysis by molecular dynamics of the reaction zone of a stationary planar detonation. In particular, we look at the influence of chemical characteristics such as the reactions reversibility and endothermicity. So, equilibrium and frozen Hugoniot of the reactive system are calculated by a specific molecular dynamics method. These results can be used to a predict the detonation characteristics such as the thermodynamic properties of ZND spike and the CJ point. We observe in particular the influence of the preliminary endothermic phase on the detonation velocity and its stability. The comparisons between these predictions and non equilibrium molecular dynamics results confirm the results of this first theoretical part. In a second step, the main hypotheses of a ZND model are extracted from the MD simulations (mainly the formalism of the reactive EOS in the reaction zone). The parameters of the corresponding model are then fitted on MD results. The final step is the implementation of the model in an hydrodynamic code. Direct comparisons between molecular dynamics simulations and hydrodynamics calculations for various 1D and 2D (in the hydrodynamics sens) configurations are presented.
Molecular dynamics simulations of microscale fluid transport
Wong, C.C.; Lopez, A.R.; Stevens, M.J.; Plimpton, S.J.
1998-02-01
Recent advances in micro-science and technology, like Micro-Electro-Mechanical Systems (MEMS), have generated a group of unique liquid flow problems that involve characteristic length scales of a Micron. Also, in manufacturing processes such as coatings, current continuum models are unable to predict microscale physical phenomena that appear in these non-equilibrium systems. It is suspected that in these systems, molecular-level processes can control the interfacial energy and viscoelastic properties at the liquid/solid boundary. A massively parallel molecular dynamics (MD) code has been developed to better understand microscale transport mechanisms, fluid-structure interactions, and scale effects in micro-domains. Specifically, this MD code has been used to analyze liquid channel flow problems for a variety of channel widths, e.g. 0.005-0.05 microns. This report presents results from MD simulations of Poiseuille flow and Couette flow problems and addresses both scaling and modeling issues. For Poiseuille flow, the numerical predictions are compared with existing data to investigate the variation of the friction factor with channel width. For Couette flow, the numerical predictions are used to determine the degree of slip at the liquid/solid boundary. Finally, the results also indicate that shear direction with respect to the wall lattice orientation can be very important. Simulation results of microscale Couette flow and microscale Poiseuille flow for two different surface structures and two different shear directions will be presented.
The Digital Material: Molecular Dynamics
NASA Astrophysics Data System (ADS)
Bailey, Nicholas P.; Cretegny, Thierry; Dolgert, Andrew J.; Myers, Christopher R.; Schiøtz, Jakob; Sethna, James P.
2001-03-01
We announce the release of the molecular dynamics component of the Digital Material. The Digital Material is our multiscale modeling software infrastructure, designed for flexibility, extensibility, and for compatibility between simulations on disparate length scales. We illustrate how we use the high-level scripting language Python to control our low-level numerical kernals, and to interface them with standard visualization and data repository tools. Our use of design-patterns methodology leads us to decompose the MD simulation into a few weakly-coupled classes, such as AtomsMover, NeighborLocator, Potential, Constraint, and BoundaryConditions.
Dynamic non-equilibrium wall-modeling for large eddy simulation at high Reynolds numbers
NASA Astrophysics Data System (ADS)
Kawai, Soshi; Larsson, Johan
2013-01-01
A dynamic non-equilibrium wall-model for large-eddy simulation at arbitrarily high Reynolds numbers is proposed and validated on equilibrium boundary layers and a non-equilibrium shock/boundary-layer interaction problem. The proposed method builds on the prior non-equilibrium wall-models of Balaras et al. [AIAA J. 34, 1111-1119 (1996)], 10.2514/3.13200 and Wang and Moin [Phys. Fluids 14, 2043-2051 (2002)], 10.1063/1.1476668: the failure of these wall-models to accurately predict the skin friction in equilibrium boundary layers is shown and analyzed, and an improved wall-model that solves this issue is proposed. The improvement stems directly from reasoning about how the turbulence length scale changes with wall distance in the inertial sublayer, the grid resolution, and the resolution-characteristics of numerical methods. The proposed model yields accurate resolved turbulence, both in terms of structure and statistics for both the equilibrium and non-equilibrium flows without the use of ad hoc corrections. Crucially, the model accurately predicts the skin friction, something that existing non-equilibrium wall-models fail to do robustly.
Dynamical Detailed Balance and Local Kms Condition for Non-Equilibrium States
NASA Astrophysics Data System (ADS)
Accardi, Luigi; Imafuku, Kentaro
The principle of detailed balance is at the basis of equilibrium physics and is equivalent to the Kubo-Martin-Schwinger (KMS) condition (under quite general assumptions). In the present paper we prove that a large class of non-equilibrium quantum systems satisfies a dynamical generalization of the detailed balance condition (dynamical detailed balance) expressing the fact that all the micro-currents, associated to the Bohr frequencies are constant. The usual (equilibrium) detailed balance condition is characterized by the property that this constant is identically zero. From this we deduce a simple and experimentally measurable relation expressing the microcurrent associated to a transition between two levels ɛm→ɛn as a linear combination of the occupation probabilities of the two levels, with coefficients given by the generalized susceptivities (transport coefficients). We then give a second characterization of the dynamical detailed balance condition using a master equation rather than the microcurrents. Finally we show that these two conditions are equivalent to a "local" generalization of the usual KMS condition. Summing up: rather than postulating some ansatz on the basis of phenomenological models or of numerical simulations, we deduce, directly in the quantum domain and from fundamental principles, some natural and simple non equilibrium generalizations of the three main characterizations of equilibrium states. Then we prove that these three, apparently very far, conditions are equivalent. These facts support our convinction that these three equivalent conditions capture a universal aspect of non equilibrium phenomena.
Relaxation to equilibrium driven via indirect control in Markovian dynamics
Romano, Raffaele
2007-11-15
We prove that it is possible to modify the stationary states of a quantum dynamical semigroup, describing the irreversible evolution of a two-level system, by means of an auxiliary two-level system, a quantum probe that can be suitably prepared. The target system and the probe can be initially entangled or uncorrelated. We find that this indirect control of the stationary states is possible, even if there are no initial correlations, under suitable conditions on the dynamical parameters characterizing the evolution of the joint system.
Non-equilibrium dynamics of coupled Luttinger liquids
NASA Astrophysics Data System (ADS)
Foini, Laura; Giamarchi, Thierry
2015-03-01
We consider the dynamics of two tunnel coupled chains after a quench in the tunneling strength is performed and the two systems are let evolve independently. We describe the form of the initial state comparing with previous results concerning the dynamics after the splitting of a one-dimensional gas of bosons into two phase coherent systems. We compute different correlation functions, among which those that are relevant for interference measurements, and discuss the emergence of effective temperatures also in connection with previous works. This work was supported in part by the Swiss NSF under Division II and ARO-MURI W911NF-14-1-0003
Universal far-from-equilibrium dynamics of a holographic superconductor.
Sonner, Julian; Del Campo, Adolfo; Zurek, Wojciech H
2015-06-23
Symmetry-breaking phase transitions are an example of non-equilibrium processes that require real-time treatment, a major challenge in strongly coupled systems without long-lived quasiparticles. Holographic duality provides such an approach by mapping strongly coupled field theories in D dimensions into weakly coupled quantum gravity in D+1 anti-de Sitter spacetime. Here we use holographic duality to study the formation of topological defects-winding numbers-in the course of a superconducting transition in a strongly coupled theory in a 1D ring. When the system undergoes the transition on a given quench time, the condensate builds up with a delay that can be deduced using the Kibble-Zurek mechanism from the quench time and the universality class of the theory, as determined from the quasinormal mode spectrum of the dual model. Typical winding numbers deposited in the ring exhibit a universal fractional power law dependence on the quench time, also predicted by the Kibble-Zurek Mechanism.
Non-Equilibrium Dynamics of Nano-channel Confined DNA: A Brownian Dynamics Simulation Study
NASA Astrophysics Data System (ADS)
Bhattacharya, Aniket; Huang, Aiqun; Reisner, Walter
We carry out Brownian dynamics (BD) simulation for a semi-flexible polymer chain characterized by a contour length Na and a persistence length lp confined inside a rectangular nanochannel to study its compression and retraction dynamics while being pushed on one end at a constant velocity by a ``nano-dozer''. We study the evolution of one dimensional concentration profile c (x , t) and the chain extension R along the channel axis (x-axis) during both the contracting as well as the retracting phases as a function of the velocity of the nano-dozer, both in steady states and in transients. Furthermore, we measure the transverse fluctuations of the chain under contraction and retraction, and the amplitude of the density profile, and compare these simulation results with those obtained from an analytical model proposed by Khorshid et al. Our studies are guided by recent experimental results by Khorshid et al. (Phys. Rev. Lett, 113, 268104 (2014)) and provide further justification to use a one dimensional PDE approach to understand the non-equilibrium dynamics of confined polymers.
Osmosis : a molecular dynamics computer simulation study
NASA Astrophysics Data System (ADS)
Lion, Thomas
Osmosis is a phenomenon of critical importance in a variety of processes ranging from the transport of ions across cell membranes and the regulation of blood salt levels by the kidneys to the desalination of water and the production of clean energy using potential osmotic power plants. However, despite its importance and over one hundred years of study, there is an ongoing confusion concerning the nature of the microscopic dynamics of the solvent particles in their transfer across the membrane. In this thesis the microscopic dynamical processes underlying osmotic pressure and concentration gradients are investigated using molecular dynamics (MD) simulations. I first present a new derivation for the local pressure that can be used for determining osmotic pressure gradients. Using this result, the steady-state osmotic pressure is studied in a minimal model for an osmotic system and the steady-state density gradients are explained using a simple mechanistic hopping model for the solvent particles. The simulation setup is then modified, allowing us to explore the timescales involved in the relaxation dynamics of the system in the period preceding the steady state. Further consideration is also given to the relative roles of diffusive and non-diffusive solvent transport in this period. Finally, in a novel modification to the classic osmosis experiment, the solute particles are driven out-of-equilibrium by the input of energy. The effect of this modification on the osmotic pressure and the osmotic ow is studied and we find that active solute particles can cause reverse osmosis to occur. The possibility of defining a new "osmotic effective temperature" is also considered and compared to the results of diffusive and kinetic temperatures..
Molecular dynamics of interface rupture
NASA Technical Reports Server (NTRS)
Koplik, Joel; Banavar, Jayanth R.
1993-01-01
Several situations have been studied in which a fluid-vapor or fluid-fluid interface ruptures, using molecular dynamics simulations of 3000 to 20,000 Lennard-Jones molecules in three dimensions. The cases studied are the Rayleigh instability of a liquid thread, the burst of a liquid drop immersed in a second liquid undergoing shear, and the rupture of a liquid sheet in an extensional flow. The late stages of the rupture process involve the gradual withdrawal of molecules from a thinning neck, or the appearance and growth of holes in a sheet. In all cases, it is found that despite the small size of the systems studied, tens of angstroms, the dynamics is in at least qualitative accord with the behavior expected from continuum calculations, and in some cases the agreement is to within tens of percent. Remarkably, this agreement occurs even though the Eulerian velocity and stress fields are essentially unmeasurable - dominated by thermal noise. The limitations and prospects for such molecular simulation techniques are assessed.
Molecular dynamics of interface rupture
NASA Astrophysics Data System (ADS)
Koplik, Joel; Banavar, Jayanth R.
1993-03-01
Several situations have been studied in which a fluid-vapor or fluid-fluid interface ruptures, using molecular dynamics simulations of 3000 to 20,000 Lennard-Jones molecules in three dimensions. The cases studied are the Rayleigh instability of a liquid thread, the burst of a liquid drop immersed in a second liquid undergoing shear, and the rupture of a liquid sheet in an extensional flow. The late stages of the rupture process involve the gradual withdrawal of molecules from a thinning neck, or the appearance and growth of holes in a sheet. In all cases, it is found that despite the small size of the systems studied, tens of angstroms, the dynamics is in at least qualitative accord with the behavior expected from continuum calculations, and in some cases the agreement is to within tens of percent. Remarkably, this agreement occurs even though the Eulerian velocity and stress fields are essentially unmeasurable - dominated by thermal noise. The limitations and prospects for such molecular simulation techniques are assessed.
Molecular dynamics of interface rupture
NASA Technical Reports Server (NTRS)
Koplik, Joel; Banavar, Jayanth R.
1993-01-01
Several situations have been studied in which a fluid-vapor or fluid-fluid interface ruptures, using molecular dynamics simulations of 3000 to 20,000 Lennard-Jones molecules in three dimensions. The cases studied are the Rayleigh instability of a liquid thread, the burst of a liquid drop immersed in a second liquid undergoing shear, and the rupture of a liquid sheet in an extensional flow. The late stages of the rupture process involve the gradual withdrawal of molecules from a thinning neck, or the appearance and growth of holes in a sheet. In all cases, it is found that despite the small size of the systems studied, tens of angstroms, the dynamics is in at least qualitative accord with the behavior expected from continuum calculations, and in some cases the agreement is to within tens of percent. Remarkably, this agreement occurs even though the Eulerian velocity and stress fields are essentially unmeasurable - dominated by thermal noise. The limitations and prospects for such molecular simulation techniques are assessed.
NASA Astrophysics Data System (ADS)
Pelissetto, Andrea; Vicari, Ettore
2017-01-01
We study the off-equilibrium behavior of systems with short-range interactions, slowly driven across a thermal first-order transition, where the equilibrium dynamics is exponentially slow. We consider a dynamics that starts in the high-T phase at time t =ti<0 and ends at t =tf>0 in the low-T phase, with a time-dependent temperature T (t )/Tc≈1 -t /ts, where ts is the protocol time scale. A general off-equilibrium scaling (OS) behavior emerges in the limit of large ts. We check it at the first-order transition of the two-dimensional q -state Potts model with q =20 and 10. The numerical results show evidence of a dynamic transition, where the OS functions show a spinodal-like singularity. Therefore, the general mean-field picture valid for systems with long-range interactions is qualitatively recovered, provided the time dependence is appropriately (logarithmically) rescaled.
NASA Astrophysics Data System (ADS)
Chiu, Mei-Hung; Chou, Chin-Cheng; Liu, Chia-Ju
2002-10-01
The purpose of this study was to investigate students' mental models of chemical equilibrium using dynamic science assessments. Research in chemical education has shown that students at various levels have misconceptions about chemical equilibrium. According to Chi's theory of conceptual change, the concept of chemical equilibrium has constraint-based features (e.g., random, simultaneous, uniform activities) that might prevent students from deeply understanding the nature of the concept of chemical equilibrium. In this study, we examined how students learned and constructed their mental models of chemical equilibrium in a cognitive apprenticeship context. Thirty 10th-grade students participated in the study: 10 in a control group and 20 in a treatment group. Both groups were presented with a series of hands-on chemical experiments. The students in the treatment group were instructed based on the main features of cognitive apprenticeship (CA), such as coaching, modeling, scaffolding, articulation, reflection, and exploration. However, the students in the control group (non-CA group) learned from the tutor without explicit CA support. The results revealed that the CA group significantly outperformed the non-CA group. The students in the CA group were capable of constructing the mental models of chemical equilibrium - including dynamic, random activities of molecules and interactions between molecules in the microworld - whereas the students in the non-CA group failed to construct similar correct mental models of chemical equilibrium. The study focuses on the process of constructing mental models, on dynamic changes, and on the actions of students (such as self-monitoring/self-correction) who are learning the concept of chemical equilibrium. Also, we discuss the implications for science education.
The "Collisions Cube" Molecular Dynamics Simulator.
ERIC Educational Resources Information Center
Nash, John J.; Smith, Paul E.
1995-01-01
Describes a molecular dynamics simulator that employs ping-pong balls as the atoms or molecules and is suitable for either large lecture halls or small classrooms. Discusses its use in illustrating many of the fundamental concepts related to molecular motion and dynamics and providing a three-dimensional perspective of molecular motion. (JRH)
Study of non-equilibrium electron dynamics in metals
NASA Astrophysics Data System (ADS)
Ibrahim, Wael Mohamed Gomaa
Thermal phenomena, such as heat propagation, lattice melting, and ablation, are the result of energy deposition in metals. A fundamental understanding of the electron dynamics leading to these thermal phenomena would benefit many laser applications, such as laser deposition of thin films and laser processing. In this work, thin metal films were prepared using the resistive heating evaporation technique. High dynamic range autocorrelators were constructed to characterize the different laser systems used in this study. The nonequilibrium electron dynamics in single layer gold films, multi-layer gold-vanadium, and gold-titanium films were studied. The time evolution of the electron temperature was monitored using femtosecond time-resolved thermoreflectivity (DeltaR/R) measurements. The validity of the Two-Temperature Model (TTM) in describing ultrafast laser heating processes was checked. The effect of the padding layer on the surface damage threshold was investigated. The experimental results revealed a reduction of the thermoreflectivity signal, DeltaRmax, for the multi-layer film that signifies a reduction in the surface electron temperature. Multi-shot damage experiments, in contrast to the thermoreflectivity measurements and the results of Qiu et al., showed no evidence of surface damage in the case of the gold sample, whereas the multi-layer sample experienced an onset of surface damage at the same experimental conditions. The suitability of the Two-Temperature Model (TTM) in describing the transport and relaxation dynamics of hot electrons accurately was verified. A new methodology for the correction of the TTM to account for the internal thermalization of the electron gas and convolution effects was achieved.
Hong, Seung Do; Ha, Man Yeong; Balachandar, S
2009-11-01
The present study investigates the variation of static contact angle of a water droplet in equilibrium with a solid surface in the absence of a body force and the dynamic contact angles of water droplet moving on a solid surface for different characteristic energies using the molecular dynamics simulation. With increasing characteristic energy, the static contact angle in equilibrium with a solid surface in the absence of a body force decreases because the hydrophobic surface changes its characteristics to the hydrophilic surface. In order to consider the effect of moving water droplet on the dynamic contact angles, we apply the constant acceleration to an individual oxygen and hydrogen atom. In the presence of a body force, the water droplet changes its shape with larger advancing contact angle than the receding angle. The dynamic contact angles are compared with the static contact angle in order to see the effect of the presence of a body force.
Bjorgaard, Josiah August; Velizhanin, Kirill A.; Tretiak, Sergei
2016-04-15
The effects of solvent on molecular processes such as excited state relaxation and photochemical reaction often occurs in a nonequilibrium regime. Dynamic processes such as these can be simulated using excited state molecular dynamics. In this paper, we describe methods of simulating nonequilibrium solvent effects in excited state molecular dynamics using linear-response time-dependent density functional theory and apparent surface charge methods. These developments include a propagation method for solvent degrees of freedom and analytical energy gradients for the calculation of forces. Finally, molecular dynamics of acetaldehyde in water or acetonitrile are demonstrated where the solute-solvent system is out of equilibrium due to photoexcitation and emission.
NASA Astrophysics Data System (ADS)
Bjorgaard, J. A.; Velizhanin, K. A.; Tretiak, S.
2016-04-01
The effects of solvent on molecular processes such as excited state relaxation and photochemical reaction often occurs in a nonequilibrium regime. Dynamic processes such as these can be simulated using excited state molecular dynamics. In this work, we describe methods of simulating nonequilibrium solvent effects in excited state molecular dynamics using linear-response time-dependent density functional theory and apparent surface charge methods. These developments include a propagation method for solvent degrees of freedom and analytical energy gradients for the calculation of forces. Molecular dynamics of acetaldehyde in water or acetonitrile are demonstrated where the solute-solvent system is out of equilibrium due to photoexcitation and emission.
Finite Temperature Quasicontinuum: Molecular Dynamics without all the Atoms
Dupuy, L; Tadmor, E B; Miller, R E; Phillips, R
2005-02-02
Using a combination of statistical mechanics and finite-element interpolation, the authors develop a coarse-grained (CG) alternative to molecular dynamics (MD) for crystalline solids at constant temperature. The new approach is significantly more efficient than MD and generalizes earlier work on the quasi-continuum method. The method is validated by recovering equilibrium properties of single crystal Ni as a function of temperature. CG dynamical simulations of nanoindentation reveal a strong dependence on temperature of the critical stress to nucleate dislocations under the indenter.
Non-equilibrium dynamics of an active colloidal ``chucker''
NASA Astrophysics Data System (ADS)
Valeriani, C.; Allen, R. J.; Marenduzzo, D.
2010-05-01
We report Monte Carlo simulations of the dynamics of a "chucker," a colloidal particle that emits smaller solute particles from its surface, isotropically and at a constant rate kc. We find that the diffusion constant of the chucker increases for small kc, as recently predicted theoretically. At large kc, the chucker diffuses more slowly due to crowding effects. We compare our simulation results to those of a "point particle" Langevin dynamics scheme in which the solute concentration field is calculated analytically, and in which hydrodynamic effects arising from colloid-solvent surface interactions can be accounted for in a coarse-grained way. By simulating the dragging of a chucker, we obtain an estimate of its apparent mobility coefficient which violates the fluctuation-dissipation theorem. We also characterize the probability density profile for a chucker which sediments onto a surface which either repels or absorbs the solute particles, and find that the steady state distributions are very different in the two cases. Our simulations are inspired by the biological example of exopolysaccharide-producing bacteria, as well as by recent experimental, simulation and theoretical work on phoretic colloidal "swimmers."
Preserving the Boltzmann ensemble in replica-exchange molecular dynamics.
Cooke, Ben; Schmidler, Scott C
2008-10-28
We consider the convergence behavior of replica-exchange molecular dynamics (REMD) [Sugita and Okamoto, Chem. Phys. Lett. 314, 141 (1999)] based on properties of the numerical integrators in the underlying isothermal molecular dynamics (MD) simulations. We show that a variety of deterministic algorithms favored by molecular dynamics practitioners for constant-temperature simulation of biomolecules fail either to be measure invariant or irreducible, and are therefore not ergodic. We then show that REMD using these algorithms also fails to be ergodic. As a result, the entire configuration space may not be explored even in an infinitely long simulation, and the simulation may not converge to the desired equilibrium Boltzmann ensemble. Moreover, our analysis shows that for initial configurations with unfavorable energy, it may be impossible for the system to reach a region surrounding the minimum energy configuration. We demonstrate these failures of REMD algorithms for three small systems: a Gaussian distribution (simple harmonic oscillator dynamics), a bimodal mixture of Gaussians distribution, and the alanine dipeptide. Examination of the resulting phase plots and equilibrium configuration densities indicates significant errors in the ensemble generated by REMD simulation. We describe a simple modification to address these failures based on a stochastic hybrid Monte Carlo correction, and prove that this is ergodic.
Nonadiabatic Molecular Dynamics with Trajectories
NASA Astrophysics Data System (ADS)
Tavernelli, Ivano
2012-02-01
In the mixed quantum-classical description of molecular systems, only the quantum character of the electronic degrees of freedom is considered while the nuclear motion is treated at a classical level. In the adiabatic case, this picture corresponds to the Born-Oppenheimer limit where the nuclei move as point charges on the potential energy surface (PES) associated with a given electronic state. Despite the success of this approximation, many physical and chemical processes do not fall in the regime where nuclei and electrons can be considered decoupled. In particular, most photoreactions pass through regions of the PES in which electron-nuclear quantum interference effects are sizeable and often crucial for a correct description of the phenomena. Recently, we have developed a trajectory-based nonadiabatic molecular dynamics scheme that describes the nuclear wavepacket as an ensemble of particles following classical trajectories on PESs derived from time-dependent density functional theory (TDDFT) [1]. The method is based on Tully's fewest switches trajectories surface hopping (TSH) where the nonadiabatic coupling elements between the different potential energy surfaces are computed on-the-fly as functionals of the ground state electron density or, equivalently, of the corresponding Kohn-Sham orbitals [2]. Here, we present the theoretical fundamentals of our approach together with an extension that allows for the direct coupling of the dynamics to an external electromagnetic field [3] as well as to the external potential generated by the environment (solvent effects) [4]. The method is applied to the study of the photodissociation dynamics of simple molecules in gas phase and to the description of the fast excited state dynamics of molecules in solution (in particular Ruthenium (II) tris(bipyridine) in water). [4pt] [1] E. Tapavicza, I. Tavernelli, U. Rothlisberger, Phys. Rev. Lett., 98, (2007) 023001. [0pt] [2] Tavernelli I.; Tapavicza E.; Rothlisberger U., J. Chem
Molecular dynamics studies on nanoscale gas transport
NASA Astrophysics Data System (ADS)
Barisik, Murat
Three-dimensional molecular dynamics (MD) simulations of nanoscale gas flows are studied to reveal surface effects. A smart wall model that drastically reduces the memory requirements of MD simulations for gas flows is introduced. The smart wall molecular dynamics (SWMD) represents three-dimensional FCC walls using only 74 wall Molecules. This structure is kept in the memory and utilized for each gas molecule surface collision. Using SWMD, fluid behavior within nano-scale confinements is studied for argon in dilute gas, dense gas, and liquid states. Equilibrium MD method is employed to resolve the density and stress variations within the static fluid. Normal stress calculations are based on the Irving-Kirkwood method, which divides the stress tensor into its kinetic and virial parts. The kinetic component recovers pressure based on the ideal gas law. The particle-particle virial increases with increased density, while the surface-particle virial develops due to the surface force field effects. Normal stresses within nano-scale confinements show anisotropy induced primarily by the surface force-field and local variations in the fluid density near the surfaces. For dilute and dense gas cases, surface-force field that extends typically 1nm from each wall induces anisotropic normal stress. For liquid case, this effect is further amplified by the density fluctuations that extend beyond the three field penetration region. Outside the wall force-field penetration and density fluctuation regions the normal stress becomes isotropic and recovers the thermodynamic pressure, provided that sufficiently large force cut-off distances are utilized in the computations. Next, non-equilibrium SWMD is utilized to investigate the surface-gas interaction effects on nanoscale shear-driven gas flows in the transition and free molecular flow regimes. For the specified surface properties and gas-surface pair interactions, density and stress profiles exhibit a universal behavior inside the
Modeling Nanocomposites for Molecular Dynamics (MD) Simulations
2015-01-01
Maximum 200 Words) The minimum energy configuration for Molecular Dynamics (MD) simulations is found for a carbon nanotube (CNT)/polymer... Carbon Nanotubes (CNTs), Molecular Dynamics Simulations 15. NUMBER OF PAGES 18 16. PRICE CODE 17. SECURITY CLASSIFICATION OF REPORT... Carbon Nanotubes ,” Macromolecules, Volume 39, Number 16, pp. 5194-5205, July 2006. 6. “VMD-Visual Molecular Dynamics ,” March 2014, http
Donnan membrane approach: from equilibrium to dynamic speciation.
Marang, Laura; Reiller, Pascal; Pepe, Monique; Benedetti, Marc F
2006-09-01
Metal ion toxicity is not simply related to the total concentration but to the free or labile metal ion concentration. One of the techniques that can measure the free metal ion concentration is the Donnan Membrane Technique (DMT). However, this method has limitations in terms of a relatively high detection limit (in relation to typical environmental concentrations) and a long equilibration time. We have therefore developed the Flux Donnan Membrane (FDM) Technique, which is a dynamic modification of the standard DMT with improved detection limits and a more rapid response. We show that experimental calibrations of metal ion transport in the FDM can be used to measure [M]free under conditions of either solution-controlled or membrane-controlled diffusive transport. The FDM was used to measure cobalt binding isotherms by a humic substance over a large range of free Co concentrations and pH.
Fire, flow and dynamic equilibrium in stream macroinvertebrate communities
Arkle, R.S.; Pilliod, D.S.; Strickler, K.
2010-01-01
The complex effects of disturbances on ecological communities can be further complicated by subsequent perturbations within an ecosystem. We investigated how wildfire interacts with annual variations in peak streamflow to affect the stability of stream macroinvertebrate communities in a central Idaho wilderness, USA. We conducted a 4-year retrospective analysis of unburned (n = 7) and burned (n = 6) catchments, using changes in reflectance values (??NBR) from satellite imagery to quantify the percentage of each catchment's riparian and upland vegetation that burned at high and low severity. For this wildland fire complex, increasing riparian burn severity and extent were associated with greater year-to-year variation, rather than a perennial increase, in sediment loads, organic debris, large woody debris (LWD) and undercut bank structure. Temporal changes in these variables were correlated with yearly peak flow in burned catchments but not in unburned reference catchments, indicating that an interaction between fire and flow can result in decreased habitat stability in burned catchments. Streams in more severely burned catchments exhibited increasingly dynamic macroinvertebrate communities and did not show increased similarity to reference streams over time. Annual variability in macroinvertebrates was attributed, predominantly, to the changing influence of sediment, LWD, riparian cover and organic debris, as quantities of these habitat components fluctuated annually depending on burn severity and annual peak streamflows. These analyses suggest that interactions among fire, flow and stream habitat may increase inter-annual habitat variability and macroinvertebrate community dynamics for a duration approaching the length of the historic fire return interval of the study area. ?? 2009 Blackwell Publishing Ltd.
Molecular dynamics simulation of benzene
NASA Astrophysics Data System (ADS)
Trumpakaj, Zygmunt; Linde, Bogumił B. J.
2016-03-01
Intermolecular potentials and a few models of intermolecular interaction in liquid benzene are tested by Molecular Dynamics (MD) simulations. The repulsive part of the Lennard-Jones 12-6 (LJ 12-6) potential is too hard, which yields incorrect results. The exp-6 potential with a too hard repulsive term is also often used. Therefore, we took an expa-6 potential with a small Gaussian correction plus electrostatic interactions. This allows to modify the curvature of the potential. The MD simulations are carried out in the temperature range 280-352 K under normal pressure and at experimental density. The Rayleigh scattering of depolarized light is used for comparison. The results of MD simulations are comparable with the experimental values.
Non-Equilibrium Dynamics Contribute to Ion Selectivity in the KcsA Channel
Haas, Stephan; Farley, Robert A.
2014-01-01
The ability of biological ion channels to conduct selected ions across cell membranes is critical for the survival of both animal and bacterial cells. Numerous investigations of ion selectivity have been conducted over more than 50 years, yet the mechanisms whereby the channels select certain ions and reject others are not well understood. Here we report a new application of Jarzynski’s Equality to investigate the mechanism of ion selectivity using non-equilibrium molecular dynamics simulations of Na+ and K+ ions moving through the KcsA channel. The simulations show that the selectivity filter of KcsA adapts and responds to the presence of the ions with structural rearrangements that are different for Na+ and K+. These structural rearrangements facilitate entry of K+ ions into the selectivity filter and permeation through the channel, and rejection of Na+ ions. A mechanistic model of ion selectivity by this channel based on the results of the simulations relates the structural rearrangement of the selectivity filter to the differential dehydration of ions and multiple-ion occupancy and describes a mechanism to efficiently select and conduct K+. Estimates of the K+/Na+ selectivity ratio and steady state ion conductance for KcsA from the simulations are in good quantitative agreement with experimental measurements. This model also accurately describes experimental observations of channel block by cytoplasmic Na+ ions, the “punch through” relief of channel block by cytoplasmic positive voltages, and is consistent with the knock-on mechanism of ion permeation. PMID:24465882
Uncertainty quantification in molecular dynamics
NASA Astrophysics Data System (ADS)
Rizzi, Francesco
This dissertation focuses on uncertainty quantification (UQ) in molecular dynamics (MD) simulations. The application of UQ to molecular dynamics is motivated by the broad uncertainty characterizing MD potential functions and by the complexity of the MD setting, where even small uncertainties can be amplified to yield large uncertainties in the model predictions. Two fundamental, distinct sources of uncertainty are investigated in this work, namely parametric uncertainty and intrinsic noise. Intrinsic noise is inherently present in the MD setting, due to fluctuations originating from thermal effects. Averaging methods can be exploited to reduce the fluctuations, but due to finite sampling, this effect cannot be completely filtered, thus yielding a residual uncertainty in the MD predictions. Parametric uncertainty, on the contrary, is introduced in the form of uncertain potential parameters, geometry, and/or boundary conditions. We address the UQ problem in both its main components, namely the forward propagation, which aims at characterizing how uncertainty in model parameters affects selected observables, and the inverse problem, which involves the estimation of target model parameters based on a set of observations. The dissertation highlights the challenges arising when parametric uncertainty and intrinsic noise combine to yield non-deterministic, noisy MD predictions of target macroscale observables. Two key probabilistic UQ methods, namely Polynomial Chaos (PC) expansions and Bayesian inference, are exploited to develop a framework that enables one to isolate the impact of parametric uncertainty on the MD predictions and, at the same time, properly quantify the effect of the intrinsic noise. Systematic applications to a suite of problems of increasing complexity lead to the observation that an uncertain PC representation built via Bayesian regression is the most suitable model for the representation of uncertain MD predictions of target observables in the
Equilibrium simulations of proteins using molecular fragment replacement and NMR chemical shifts.
Boomsma, Wouter; Tian, Pengfei; Frellsen, Jes; Ferkinghoff-Borg, Jesper; Hamelryck, Thomas; Lindorff-Larsen, Kresten; Vendruscolo, Michele
2014-09-23
Methods of protein structure determination based on NMR chemical shifts are becoming increasingly common. The most widely used approaches adopt the molecular fragment replacement strategy, in which structural fragments are repeatedly reassembled into different complete conformations in molecular simulations. Although these approaches are effective in generating individual structures consistent with the chemical shift data, they do not enable the sampling of the conformational space of proteins with correct statistical weights. Here, we present a method of molecular fragment replacement that makes it possible to perform equilibrium simulations of proteins, and hence to determine their free energy landscapes. This strategy is based on the encoding of the chemical shift information in a probabilistic model in Markov chain Monte Carlo simulations. First, we demonstrate that with this approach it is possible to fold proteins to their native states starting from extended structures. Second, we show that the method satisfies the detailed balance condition and hence it can be used to carry out an equilibrium sampling from the Boltzmann distribution corresponding to the force field used in the simulations. Third, by comparing the results of simulations carried out with and without chemical shift restraints we describe quantitatively the effects that these restraints have on the free energy landscapes of proteins. Taken together, these results demonstrate that the molecular fragment replacement strategy can be used in combination with chemical shift information to characterize not only the native structures of proteins but also their conformational fluctuations.
de Picciotto, Seymour; Imperiali, Barbara; Griffith, Linda G.; Wittrup, K. Dane
2014-01-01
Reagentless biosensors rely on the interaction of a binding partner and its target to generate a change in fluorescent signal using an environment sensitive fluorophore or Förster Resonance Energy Transfer. Binding affinity can exert a significant influence on both the equilibrium and the dynamic response characteristics of such a biosensor. We here develop a kinetic model for the dynamic performance of a reagentless biosensor. Using a sinusoidal signal for ligand concentration, our findings suggest that it is optimal to use a binding moiety whose equilibrium dissociation constant matches that of the average predicted input signal, while maximizing both the association rate constant and the dissociation rate constant at the necessary ratio to create the desired equilibrium constant. Although practical limitations constrain the attainment of these objectives, the derivation of these design principles provides guidance for improved reagentless biosensor performance and metrics for quality standards in the development of biosensors. These concepts are broadly relevant to reagentless biosensor modalities. PMID:24814226
Ultrafast dynamics of fluctuations in high-temperature superconductors far from equilibrium.
Perfetti, L; Sciolla, B; Biroli, G; van der Beek, C J; Piovera, C; Wolf, M; Kampfrath, T
2015-02-13
Despite extensive work on high-temperature superconductors, the critical behavior of an incipient condensate has so far been studied exclusively under equilibrium conditions. Here, we excite Bi(2)Sr(2)CaCu(2)O(8+δ) with a femtosecond laser pulse and monitor the subsequent nonequilibrium dynamics of the midinfrared conductivity. Our data allow us to discriminate temperature regimes where superconductivity is either coherent, fluctuating or vanishingly small. Above the transition temperature T(c), we make the striking observation that the relaxation to equilibrium exhibits power-law dynamics and scaling behavior, both for optimally and underdoped superconductors. Our findings can in part be modeled using time-dependent Ginzburg-Landau theory, and they provide strong indication of universality in systems far from equilibrium.
de Picciotto, Seymour; Imperiali, Barbara; Griffith, Linda G; Wittrup, K Dane
2014-09-01
Reagentless biosensors rely on the interaction of a binding partner and its target to generate a change in fluorescent signal using an environment-sensitive fluorophore or Förster resonance energy transfer. Binding affinity can exert a significant influence on both the equilibrium and the dynamic response characteristics of such a biosensor. We here develop a kinetic model for the dynamic performance of a reagentless biosensor. Using a sinusoidal signal for ligand concentration, our findings suggest that it is optimal to use a binding moiety whose equilibrium dissociation constant matches that of the average predicted input signal, while maximizing both the association rate constant and the dissociation rate constant at the necessary ratio to create the desired equilibrium constant. Although practical limitations constrain the attainment of these objectives, the derivation of these design principles provides guidance for improved reagentless biosensor performance and metrics for quality standards in the development of biosensors. These concepts are broadly relevant to reagentless biosensor modalities.
Molecular Dynamics Simulations of Perylenediimide DNA Base Surrogates.
Markegard, Cade B; Mazaheripour, Amir; Jocson, Jonah-Micah; Burke, Anthony M; Dickson, Mary N; Gorodetsky, Alon A; Nguyen, Hung D
2015-09-03
Perylene-3,4,9,10-tetracarboxylic diimides (PTCDIs) are a well-known class of organic materials. Recently, these molecules have been incorporated within DNA as base surrogates, finding ready applications as probes of DNA structure and function. However, the assembly dynamics and kinetics of PTCDI DNA base surrogates have received little attention to date. Herein, we employ constant temperature molecular dynamics simulations to gain an improved understanding of the assembly of PTCDI dimers and trimers. We also use replica-exchange molecular dynamics simulations to elucidate the energetic landscape dictating the formation of stacked PTCDI structures. Our studies provide insight into the equilibrium configurations of multimeric PTCDIs and hold implications for the construction of DNA-inspired systems from perylene-derived organic semiconductor building blocks.
Emergent Eigenstate Solution to Quantum Dynamics Far from Equilibrium
NASA Astrophysics Data System (ADS)
Vidmar, Lev; Iyer, Deepak; Rigol, Marcos
2017-04-01
The quantum dynamics of interacting many-body systems has become a unique venue for the realization of novel states of matter. Here, we unveil a new class of nonequilibrium states that are eigenstates of an emergent local Hamiltonian. The latter is explicitly time dependent and, even though it does not commute with the physical Hamiltonian, it behaves as a conserved quantity of the time-evolving system. We discuss two examples of integrable systems in which the emergent eigenstate solution can be applied for an extensive (in system size) time: transport in one-dimensional lattices with initial particle (or spin) imbalance and sudden expansion of quantum gases in optical lattices. We focus on noninteracting spinless fermions, hard-core bosons, and the Heisenberg model. We show that current-carrying states can be ground states of emergent local Hamiltonians, and that they can exhibit a quasimomentum distribution function that is peaked at nonzero (and tunable) quasimomentum. We also show that time-evolving states can be highly excited eigenstates of emergent local Hamiltonians, with an entanglement entropy that does not exhibit volume-law scaling.
Molecular dynamics of membrane proteins.
Woolf, Thomas B.; Crozier, Paul Stewart; Stevens, Mark Jackson
2004-10-01
Understanding the dynamics of the membrane protein rhodopsin will have broad implications for other membrane proteins and cellular signaling processes. Rhodopsin (Rho) is a light activated G-protein coupled receptor (GPCR). When activated by ligands, GPCRs bind and activate G-proteins residing within the cell and begin a signaling cascade that results in the cell's response to external stimuli. More than 50% of all current drugs are targeted toward G-proteins. Rho is the prototypical member of the class A GPCR superfamily. Understanding the activation of Rho and its interaction with its Gprotein can therefore lead to a wider understanding of the mechanisms of GPCR activation and G-protein activation. Understanding the dark to light transition of Rho is fully analogous to the general ligand binding and activation problem for GPCRs. This transition is dependent on the lipid environment. The effect of lipids on membrane protein activity in general has had little attention, but evidence is beginning to show a significant role for lipids in membrane protein activity. Using the LAMMPS program and simulation methods benchmarked under the IBIG program, we perform a variety of allatom molecular dynamics simulations of membrane proteins.
ERIC Educational Resources Information Center
Cloonan, Carrie A.; Andrew, Julie A.; Nichol, Carolyn A.; Hutchinson, John S.
2011-01-01
This article describes an activity that can be used as an inquiry-based laboratory or demonstration for either high school or undergraduate chemistry students to provide a basis for understanding both vapor pressure and the concept of dynamic phase equilibrium. The activity includes a simple setup to create a closed system of only water liquid and…
ERIC Educational Resources Information Center
Cloonan, Carrie A.; Andrew, Julie A.; Nichol, Carolyn A.; Hutchinson, John S.
2011-01-01
This article describes an activity that can be used as an inquiry-based laboratory or demonstration for either high school or undergraduate chemistry students to provide a basis for understanding both vapor pressure and the concept of dynamic phase equilibrium. The activity includes a simple setup to create a closed system of only water liquid and…
Equilibrium and kinetics of water adsorption in carbon molecular sieve: theory and experiment.
Rutherford, S W; Coons, J E
2004-09-28
Measurements of water adsorption equilibrium and kinetics in Takeda carbon molecular sieve (CMS) were undertaken in an effort to characterize fundamental mechanisms of adsorption and transport. Adsorption equilibrium revealed a type III isotherm that was characterized by cooperative multimolecular sorption theory. Water adsorption was found to be reversible and did not display hysteresis upon desorption over the conditions studied. Adsorption kinetics measurements revealed that a Fickian diffusion mechanism governed the uptake of water and that the rate of adsorption decreased with increasing relative pressure. Previous investigations have attributed the observed decreasing trend in the rate of adsorption to blocking of micropores. Here, it is proposed that the decrease is attributed to the thermodynamic correction to Fick's law which is formulated on the basis of the chemical potential as the driving force for transport. The thermodynamically corrected formulation accounted for observations of transport of water and other molecules in CMS.
Vapor-liquid equilibrium of ethanol-water system in the presence of molecular sieves
Abu Al-Rub, F.A.; Banat, F.A.; Jumah, R.
1999-09-01
Adsorptive distillation is a new process to separate liquid mixtures in a packed distillation column. It depends on using active packing material instead of inert packing material in a packed distillation column. The active packing material can affect the intermolecular forces among the system components and thus alter its vapor-liquid equilibrium (VLE). The VLE of the ethanol-water system at 1 atm was studied using a circulation still in the absence and in the presence of different amounts of 4 {angstrom} molecular sieves. The results obtained showed that the VLE of the system was altered in the presence of the molecular sieves, the azeotropic point of the system (at 89.7 mol% ethanol in the normal case) was eliminated and considerable separation was achieved for a mixture of azeotropic composition, and the alteration in the VLE of a given binary mixture is a function of the pore size and the amount of the molecular sieves.
Non-equilibrium dynamics and floral trait interactions shape extant angiosperm diversity
O'Meara, Brian C.; Smith, Stacey D.; Armbruster, W. Scott; Harder, Lawrence D.; Hardy, Christopher R.; Hileman, Lena C.; Hufford, Larry; Litt, Amy; Magallón, Susana; Smith, Stephen A.; Stevens, Peter F.; Fenster, Charles B.; Diggle, Pamela K.
2016-01-01
Why are some traits and trait combinations exceptionally common across the tree of life, whereas others are vanishingly rare? The distribution of trait diversity across a clade at any time depends on the ancestral state of the clade, the rate at which new phenotypes evolve, the differences in speciation and extinction rates across lineages, and whether an equilibrium has been reached. Here we examine the role of transition rates, differential diversification (speciation minus extinction) and non-equilibrium dynamics on the evolutionary history of angiosperms, a clade well known for the abundance of some trait combinations and the rarity of others. Our analysis reveals that three character states (corolla present, bilateral symmetry, reduced stamen number) act synergistically as a key innovation, doubling diversification rates for lineages in which this combination occurs. However, this combination is currently less common than predicted at equilibrium because the individual characters evolve infrequently. Simulations suggest that angiosperms will remain far from the equilibrium frequencies of character states well into the future. Such non-equilibrium dynamics may be common when major innovations evolve rarely, allowing lineages with ancestral forms to persist, and even outnumber those with diversification-enhancing states, for tens of millions of years. PMID:27147092
Non-equilibrium dynamics and floral trait interactions shape extant angiosperm diversity.
O'Meara, Brian C; Smith, Stacey D; Armbruster, W Scott; Harder, Lawrence D; Hardy, Christopher R; Hileman, Lena C; Hufford, Larry; Litt, Amy; Magallón, Susana; Smith, Stephen A; Stevens, Peter F; Fenster, Charles B; Diggle, Pamela K
2016-05-11
Why are some traits and trait combinations exceptionally common across the tree of life, whereas others are vanishingly rare? The distribution of trait diversity across a clade at any time depends on the ancestral state of the clade, the rate at which new phenotypes evolve, the differences in speciation and extinction rates across lineages, and whether an equilibrium has been reached. Here we examine the role of transition rates, differential diversification (speciation minus extinction) and non-equilibrium dynamics on the evolutionary history of angiosperms, a clade well known for the abundance of some trait combinations and the rarity of others. Our analysis reveals that three character states (corolla present, bilateral symmetry, reduced stamen number) act synergistically as a key innovation, doubling diversification rates for lineages in which this combination occurs. However, this combination is currently less common than predicted at equilibrium because the individual characters evolve infrequently. Simulations suggest that angiosperms will remain far from the equilibrium frequencies of character states well into the future. Such non-equilibrium dynamics may be common when major innovations evolve rarely, allowing lineages with ancestral forms to persist, and even outnumber those with diversification-enhancing states, for tens of millions of years.
Studies of thermal transport properties using molecular dynamics simulation techniques
NASA Astrophysics Data System (ADS)
Ratanapisit, Juraivan
The purpose of this research has been to investigate the transport properties of fluids using novel techniques in molecular dynamics simulations: symplectic integration algorithms for equations of motion, Baranyai's thermostatted fluid wall algorithm, and Rapaport's algorithm for hard chain fluids. In the symplectic integration study, an extensive series of equilibrium molecular dynamic simulations have been performed to investigate the accuracy, stability and efficiency of second order explicit symplectic integrators: position Verlet, velocity Verlet, and the McLauchlan-Atela algorithms. To our knowledge, previous studies of the symplectic integrators have only looked at the thermodynamic energy using a simple model fluid. Our work presents realistic but perhaps the simplest simulations possible to test the effect of the integrators on the three main transport properties. Our results suggest that if an algorithm fails to adequately conserve energy, it will also show significant uncertainties in transport property calculations. A large portion of the simulation study focused on a new algorithm for thermal conductivity based on Baranyai's fluid wall method. This algorithm is stable enough to perform simulations even using large time steps and provides reasonable values and uncertainties for the thermal conductivity. The investigation was conducted using two different thermostat algorithms: the Gaussian and Nosé-Hoover thermostats. The final part of this research focused on the viscosity of hard chain fluids. This study was initiated with an investigation of the equilibrium molecular dynamic simulations of pure hard-sphere molecules. The natural extension of that work was to hard chain fluids. (Abstract shortened by UMI.)
NASA Astrophysics Data System (ADS)
Yamakoshi, Tomotake; Watanabe, Shinichi; Zhang, Chen; Greene, Chris H.
2013-05-01
The ultracold molecular conversion rate occurring in an adiabatic ramp through a Fano-Feshbach resonance is studied and compared in two statistical models. One model, the so-called stochastic phase-space sampling (SPSS) [Hodby , Phys. Rev. Lett.PRLTAO0031-900710.1103/PhysRevLett.94.120402 94, 120402 (2005)] evaluates the overlap of two atomic distributions in phase space by sampling atomic pairs according to a phase-space criterion. The other model, the chemical equilibrium theory (ChET) [Watabe and Nikuni, Phys. Rev. APLRAAN1050-294710.1103/PhysRevA.77.013616 77, 013616 (2008)] considers atomic and molecular distributions in the limit of the chemical and thermal equilibrium. The present study applies SPSS and ChET to a prototypical system of K+K→ K2 in all the symmetry combinations, namely Fermi-Fermi, Bose-Bose, and Bose-Fermi cases. To examine implications of the phase-space criterion for SPSS, the behavior of molecular conversion is analyzed using four distinct geometrical constraints. Our comparison of the results of SPSS with those of ChET shows that while they appear similar in most situations, the two models give rise to rather dissimilar behaviors when the presence of a Bose-Einstein condensate strongly affects the molecule formation.
Non-equilibrium dynamics of atomic Fermi and Bose gas under lattice geometry transformation
NASA Astrophysics Data System (ADS)
Lai, Chen-Yen; Chien, Chih-Chun
2015-03-01
The tunability of ultra-cold atom experiments has provide a new arena of exploring quantum effect in both bosonic and fermonic system in and out of equilibrium. According to recent experiments, a triangular lattice can be dynamically tuned into a square or kagome lattice by adjusting frequency and focus point of laser beams. We simulate the dynamical properties of single component fermions and weakly interacting bosons under various transformation processes, including different ramping time scales, different ramping functions, and more importantly into different types of lattice geometry. A non-equilibrium steady state, which is not thermalized, is found in single component fermion system under different particle densities in both small size system and in the thermodynamic limit. In contras, weakly interacting bosons do not exhibit observable steady state behavior. This opens new opportunities of research on dynamical multi-band effects.
The new physics of non-equilibrium condensates: insights from classical dynamics.
Eastham, P R
2007-07-25
We discuss the dynamics of classical Dicke-type models, aiming to clarify the mechanisms by which coherent states could develop in potentially non-equilibrium systems such as semiconductor microcavities. We present simulations of an undamped model which show spontaneous coherent states with persistent oscillations in the magnitude of the order parameter. These states are generalizations of superradiant ringing to the case of inhomogeneous broadening. They correspond to the persistent gap oscillations proposed in fermionic atomic condensates, and arise from a variety of initial conditions. We show that introducing randomness into the couplings can suppress the oscillations, leading to a limiting dynamics with a time-independent order parameter. This demonstrates that non-equilibrium generalizations of polariton condensates can be created even without dissipation. We explain the dynamical origins of the coherence in terms of instabilities of the normal state, and consider how it can additionally develop through scattering and dissipation.
Dynamical Localization in Molecular Systems.
NASA Astrophysics Data System (ADS)
Wang, Xidi
In the first four chapters of this thesis we concentrate on the Davydov model which describes the vibrational energy quanta of Amide I bonds (C=O bonds on the alpha -helix) coupled to the acoustic phonon modes of the alpha-helix backbone in the form of a Frohlich Hamiltonian. Following a brief introduction in chapter one, in chapter two we formulate the dynamics of vibrational quanta at finite temperature by using coherent state products. The fluctuation-dissipation relation is derived. At zero temperature, in the continuum limit, we recover the original results of Davydov. We also achieve good agreement with numerical simulations. In chapter three, the net contraction of the lattice is calculated exactly at any temperature, and its relation to the so -call "topological stability" of the Davydov soliton is discussed. In the second section of the chapter three we calculate the overtone spectra of crystalline acetanilide (according to some opinions ACN provides experimental evidence for the existence of Davydov solitons). Good agreement with experimental data has been obtained. In chapter four we study the self-trapped vibrational excitations by the Quantum Monte Carlo technique. For a single excitation, the temperature dependence of different physical observables is calculated. The quasi-particle which resembles the Davydov soliton has been found to be fairly narrow using the most commonly used data for the alpha -helix; at temperatures above a few Kelvin, the quasi-particle reaches its smallest limit (extends over three sites), which implies diffusive motion of the small polaron-like quasi-particle at high temperatures. For the multi-excitation case, bound pairs and clusters of excitations are found at low temperatures; they gradually dissociate when the temperature of the system is increased as calculated from the density-density correlation function. In the last chapter of this thesis, we study a more general model of dynamical local modes in molecular systems
NASA Technical Reports Server (NTRS)
Tiwari, S. N.; Jha, M. K.
1993-01-01
Basic formulations, analyses, and numerical procedures are presented to investigate radiative heat interactions in diatomic and polyatomic gases under local and nonlocal thermodynamic equilibrium conditions. Essential governing equations are presented for both gray and nongray gases. Information is provided on absorption models, relaxation times, and transfer equations. Radiative flux equations are developed which are applicable under local and nonlocal thermodynamic equilibrium conditions. The problem is solved for fully developed laminar incompressible flows between two parallel plates under the boundary condition of a uniform surface heat flux. For specific applications, three diatomic and three polyatomic gases are considered. The results are obtained numerically by employing the method of variation of parameters. The results are compared under local and nonlocal thermodynamic equilibrium conditions at different temperature and pressure conditions. Both gray and nongray studies are conducted extensively for all molecular gases considered. The particular gases selected for this investigation are CO, NO, OH, CO2, H2O, and CH4. The temperature and pressure range considered are 300-2000 K and 0.1-10 atmosphere, respectively. In general, results demonstrate that the gray gas approximation overestimates the effect of radiative interaction for all conditions. The conditions of NLTE, however, result in underestimation of radiative interactions. The method developed for this study can be extended to solve complex problems of radiative heat transfer involving nonequilibrium phenomena.
Piecewise Smooth Dynamical Systems Theory: The Case of the Missing Boundary Equilibrium Bifurcations
NASA Astrophysics Data System (ADS)
Hogan, S. J.; Homer, M. E.; Jeffrey, M. R.; Szalai, R.
2016-10-01
We present two codimension-one bifurcations that occur when an equilibrium collides with a discontinuity in a piecewise smooth dynamical system. These simple cases appear to have escaped recent classifications. We present them here to highlight some of the powerful results from Filippov's book Differential Equations with Discontinuous Righthand Sides (Kluwer, 1988). Filippov classified the so-called boundary equilibrium collisions without providing their unfolding. We show the complete unfolding here, for the first time, in the particularly interesting case of a node changing its stability as it collides with a discontinuity. We provide a prototypical model that can be used to generate all codimension-one boundary equilibrium collisions, and summarize the elements of Filippov's work that are important in achieving a full classification.
Collective Flocking Dynamics: Long Rang Order in a Non-Equilibrium 2D XY Model
NASA Astrophysics Data System (ADS)
Tu, Yuhai
1996-03-01
We propose and study a non-equilibrium continuum dynamical model for the collective motion of large groups of biological organisms (e.g., flocks of birds, slime molds, schools of fishs, etc.) (J. Toner and Y. Tu, Phys. Rev. Lett.), 75(23), 4326(1995) Our model becomes highly non-trivial, and different from the equilibrium model, for d
Probing equilibrium by nonequilibrium dynamics: Aging in Co/Cr superlattices
NASA Astrophysics Data System (ADS)
Mukherjee, T.; Pleimling, M.; Binek, Ch.
2010-10-01
Magnetization relaxation is investigated in a structurally ordered magnetic Co/Cr superlattice. Tailored nanoscale periodicity creates mesoscopic spatial magnetic correlations with slow relaxation dynamics when quenching the system into a nonequilibrium state. Magnetization transients are measured after exposing the heterostructure to a magnetic set field for various waiting times. Scaling analysis reveals an asymptotic power-law behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in nonequilibrium properties. Our findings suggest scaling analysis of nonequilibrium data as a probe for weak equilibrium phase transitions.
Molecular Dynamics Simulation of Supercritical Spray Phenomena
2008-09-26
Dynamics of the Rheological and Structural Properties of Linear and Branched Molecules. Simple Shear and Poiseuille Flows ; Instabilities and Slip...Michael Barrucco Publications: "Comparison of Wall Models for the Molecular Dynamics Simulation of Micro flows ," R. D. Branam and M. M...Performance 3. DATES COVERED (From - To) 1 Dec. 2003 - 31 May 2008 4. TITLE AND SUBTITLE Molecular Dynamics Simulation of Supercritical
Sedimentation dynamics and equilibrium profiles in multicomponent mixtures of colloidal particles.
Spruijt, E; Biesheuvel, P M
2014-02-19
In this paper we give a general theoretical framework that describes the sedimentation of multicomponent mixtures of particles with sizes ranging from molecules to macroscopic bodies. Both equilibrium sedimentation profiles and the dynamic process of settling, or its converse, creaming, are modeled. Equilibrium profiles are found to be in perfect agreement with experiments. Our model reconciles two apparently contradicting points of view about buoyancy, thereby resolving a long-lived paradox about the correct choice of the buoyant density. On the one hand, the buoyancy force follows necessarily from the suspension density, as it relates to the hydrostatic pressure gradient. On the other hand, sedimentation profiles of colloidal suspensions can be calculated directly using the fluid density as apparent buoyant density in colloidal systems in sedimentation-diffusion equilibrium (SDE) as a result of balancing gravitational and thermodynamic forces. Surprisingly, this balance also holds in multicomponent mixtures. This analysis resolves the ongoing debate of the correct choice of buoyant density (fluid or suspension): both approaches can be used in their own domain. We present calculations of equilibrium sedimentation profiles and dynamic sedimentation that show the consequences of these insights. In bidisperse mixtures of colloids, particles with a lower mass density than the homogeneous suspension will first cream and then settle, whereas particles with a suspension-matched mass density form transient, bimodal particle distributions during sedimentation, which disappear when equilibrium is reached. In all these cases, the centers of the distributions of the particles with the lowest mass density of the two, regardless of their actual mass, will be located in equilibrium above the so-called isopycnic point, a natural consequence of their hard-sphere interactions. We include these interactions using the Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) equation of
Full characterization of GPCR monomer-dimer dynamic equilibrium by single molecule imaging.
Kasai, Rinshi S; Suzuki, Kenichi G N; Prossnitz, Eric R; Koyama-Honda, Ikuko; Nakada, Chieko; Fujiwara, Takahiro K; Kusumi, Akihiro
2011-02-07
Receptor dimerization is important for many signaling pathways. However, the monomer-dimer equilibrium has never been fully characterized for any receptor with a 2D equilibrium constant as well as association/dissociation rate constants (termed super-quantification). Here, we determined the dynamic equilibrium for the N-formyl peptide receptor (FPR), a chemoattractant G protein-coupled receptor (GPCR), in live cells at 37°C by developing a single fluorescent-molecule imaging method. Both before and after liganding, the dimer-monomer 2D equilibrium is unchanged, giving an equilibrium constant of 3.6 copies/µm(2), with a dissociation and 2D association rate constant of 11.0 s(-1) and 3.1 copies/µm(2)s(-1), respectively. At physiological expression levels of ∼2.1 receptor copies/µm(2) (∼6,000 copies/cell), monomers continually convert into dimers every 150 ms, dimers dissociate into monomers in 91 ms, and at any moment, 2,500 and 3,500 receptor molecules participate in transient dimers and monomers, respectively. Not only do FPR dimers fall apart rapidly, but FPR monomers also convert into dimers very quickly.
Hele, Timothy J H; Willatt, Michael J; Muolo, Andrea; Althorpe, Stuart C
2015-05-21
We recently obtained a quantum-Boltzmann-conserving classical dynamics by making a single change to the derivation of the "Classical Wigner" approximation. Here, we show that the further approximation of this "Matsubara dynamics" gives rise to two popular heuristic methods for treating quantum Boltzmann time-correlation functions: centroid molecular dynamics (CMD) and ring-polymer molecular dynamics (RPMD). We show that CMD is a mean-field approximation to Matsubara dynamics, obtained by discarding (classical) fluctuations around the centroid, and that RPMD is the result of discarding a term in the Matsubara Liouvillian which shifts the frequencies of these fluctuations. These findings are consistent with previous numerical results and give explicit formulae for the terms that CMD and RPMD leave out.
Schuck, Peter; Gillis, Richard B.; Besong, Tabot M.D.; Almutairi, Fahad; Adams, Gary G.; Rowe, Arthur J.; Harding, Stephen E.
2014-01-01
Sedimentation equilibrium (analytical ultracentrifugation) is one of the most inherently suitable methods for the determination of average molecular weights and molecular weight distributions of polymers, because of its absolute basis (no conformation assumptions) and inherent fractionation ability (without the need for columns or membranes and associated assumptions over inertness). With modern instrumentation it is also possible to run up to 21 samples simultaneously in a single run. Its application has been severely hampered because of difficulties in terms of baseline determination (incorporating estimation of the concentration at the air/solution meniscus) and complexity of the analysis procedures. We describe a new method for baseline determination based on a smart-smoothing principle and built into the highly popular platform SEDFIT for the analysis of the sedimentation behavior of natural and synthetic polymer materials. The SEDFIT-MSTAR procedure – which takes only a few minutes to perform - is tested with four synthetic data sets (including a significantly non-ideal system) a naturally occurring protein (human IgG1) and two naturally occurring carbohydrate polymers (pullulan and λ–carrageenan) in terms of (i) weight average molecular weight for the whole distribution of species in the sample (ii) the variation in “point” average molecular weight with local concentration in the ultracentrifuge cell and (iii) molecular weight distribution. PMID:24244936
Schuck, Peter; Gillis, Richard B; Besong, Tabot M D; Almutairi, Fahad; Adams, Gary G; Rowe, Arthur J; Harding, Stephen E
2014-01-07
Sedimentation equilibrium (analytical ultracentrifugation) is one of the most inherently suitable methods for the determination of average molecular weights and molecular weight distributions of polymers, because of its absolute basis (no conformation assumptions) and inherent fractionation ability (without the need for columns or membranes and associated assumptions over inertness). With modern instrumentation it is also possible to run up to 21 samples simultaneously in a single run. Its application has been severely hampered because of difficulties in terms of baseline determination (incorporating estimation of the concentration at the air/solution meniscus) and complexity of the analysis procedures. We describe a new method for baseline determination based on a smart-smoothing principle and built into the highly popular platform SEDFIT for the analysis of the sedimentation behavior of natural and synthetic polymer materials. The SEDFIT-MSTAR procedure - which takes only a few minutes to perform - is tested with four synthetic data sets (including a significantly non-ideal system), a naturally occurring protein (human IgG1) and two naturally occurring carbohydrate polymers (pullulan and λ-carrageenan) in terms of (i) weight average molecular weight for the whole distribution of species in the sample (ii) the variation in "point" average molecular weight with local concentration in the ultracentrifuge cell and (iii) molecular weight distribution.
Pal, Krishnendu; Gangopadhyay, Gautam
2016-11-01
Inactivation path of voltage gated sodium channel has been studied here under various voltage protocols as it is the main governing factor for the periodic occurrence and shape of the action potential. These voltage protocols actually serve as non-equilibrium response spectroscopic tools to study the ion channel in non-equilibrium environment. In contrast to a lot of effort in finding the crystal structure based molecular mechanism of closed-state(CSI) and open-state inactivation(OSI); here our approach is to understand the dynamical characterization of inactivation. The kinetic flux as well as energetic contribution of the closed and open- state inactivation path is compared here for voltage protocols, namely constant, pulsed and oscillating. The non-equilibrium thermodynamic quantities used in response to these voltage protocols serve as improved characterization tools for theoretical understanding which not only agrees with the previously known kinetic measurements but also predict the energetically optimum processes to sustain the auto-regulatory mechanism of action potential and the consequent inactivation steps needed. The time dependent voltage pattern governs the population of the conformational states which when couple with characteristic rate parameters, the CSI and OSI selectivity arise dynamically to control the inactivation path. Using constant, pulsed and continuous oscillating voltage protocols we have shown that during depolarization the OSI path is more favored path of inactivation however, in the hyper-polarized situation the CSI is favored. It is also shown that the re-factorisation of inactivated sodium channel to resting state occurs via CSI path. Here we have shown how the subtle energetic and entropic cost due to the change in the depolarization magnitude determines the optimum path of inactivation. It is shown that an efficient CSI and OSI dynamical profile in principle can characterize the open-state drug blocking phenomena.
Molecular Dynamics Simulations of Acoustic Properties of Colloidal Suspensions
NASA Astrophysics Data System (ADS)
Noguez, Cecilia; Esquivel-Sirvent, Raul; Ramírez-Santiago, Guillermo
1998-03-01
Recent experiments of ultrasound waves in colloidal suspensions [1] have found that the acoustic velocity and attenuation exhibit an anomalous behavior close to the solid volume concentration of 40%. Currently, there appears that there is no clear understanding of these results. Motivated by these observations we have carried out extensive non-equilibrium molecular dynamics simulations to study the propagation of pressure waves through a colloidal suspension. The simulations consider the far from equilibrium corrections and calculate the viscosity and attenuation of sound waves traveling in the suspension. These quantities are studied as functions of frecuency and volume fraction. The possible relation between the results from the simulations and the experimental observatios is briefly discussed. [1] R. Esquivel-Sirvent and D. H. Green, Appl. Phys. Lett. 67, 3087 (1995); ibid, Mat. Res. Soc. Symp. 407, p. 99 (1996).
Gliding flight in snakes: non-equilibrium trajectory dynamics and kinematics
NASA Astrophysics Data System (ADS)
Socha, Jake; Miklasz, Kevin; Jafari, Farid; Vlachos, Pavlos
2010-11-01
For animal gliders that live in trees, a glide trajectory begins in free fall and, given sufficient space, transitions to equilibrium gliding with no net forces on the body. However, the dynamics of non-equilibrium gliding are not well understood. Of any terrestrial animal glider, snakes may exhibit the most complicated glide patterns resulting from their highly active undulatory behavior. Our aim was to determine the characteristics of snake gliding during the transition to equilibrium. We launched "flying" snakes (Chrysopelea paradisi) from a 15 m tower and recorded the mid-to-end portion of trajectories with four videocameras to reconstruct the snake's 3D body position. Additionally, we developed a simple analytical model of gliding assuming only steady-state forces of lift, drag and weight acting on the body and used it to explore effects of wing loading, lift-to-drag ratio, and initial velocity on trajectory dynamics. Despite the vertical space provided to transition to steady-state gliding, snakes did not exhibit equilibrium gliding and in fact displayed a net positive acceleration in the vertical axis.
Molecular-dynamics simulation of thermal conductivity in amorphous silicon
NASA Astrophysics Data System (ADS)
Lee, Young Hee; Biswas, R.; Soukoulis, C. M.; Wang, C. Z.; Chan, C. T.; Ho, K. M.
1991-03-01
The temperature-dependent thermal conductivity κ(T) of amorphous silicon has been calculated from equilibrium molecular-dynamics simulations using the time correlations of the heat flux operator in which anharmonicity is explicitly incorporated. The Stillinger-Weber two- and three-body Si potential and the Wooten-Weaire-Winer a-Si model were utilized. The calculations correctly predict an increasing thermal conductivity at low temperatures (below 400 K). The κ(T), for T>400 K, is affected by the thermally generated coordination-defect states. Comparisons to both experiment and previous calculations will be described.
NASA Astrophysics Data System (ADS)
Pinto, Oscar A.; Romá, Federico; Bustingorry, Sebastian
2014-12-01
We study the critical behavior and the out-of-equilibrium dynamics of a two-dimensional Ising model with non-static interactions. In our model, bonds are dynamically changing according to a majority rule depending on the set of closest neighbors of each spin pair, which prevents the system from ordering in a full ferromagnetic or antiferromagnetic state. Using a parallel-tempering Monte Carlo algorithm, we find that the model undergoes a continuous phase transition at finite temperature, which belongs to the Ising universality class. The properties of the bond structure and the ground-state entropy are also studied. Finally, we analyze the out-of-equilibrium dynamics which displays typical glassy characteristics at a temperature well below the critical one.
Calculation of transport properties of liquid metals and their alloys via molecular dynamics
NASA Astrophysics Data System (ADS)
Cherne, Frank Joseph, III
The advanced casting modeler requires accurate viscosity and diffusivity data of liquid metals and their alloys. The present work discusses the use of equilibrium and non-equilibrium molecular dynamics techniques to obtain such data without having to rely on oversimplified phenomenological expressions or difficult and expensive experiments. Utilizing the embedded atom method (EAM), the viscosities and diffusivities for a series of equilibrium and non-equilibrium molecular dynamics simulations of nickel, aluminum, and nickel-aluminum alloys are presented. A critical comparison between the equilibrium and non-equilibrium methods is presented. Besides the transport properties, structural data for the liquids are also evaluated. EAM does a poor job of describing the transport properties of nickel-aluminum alloys, particularly near the equiatomic concentration. It has been suggested that charge transfer between nickel and aluminum atoms is responsible for the discrepancy between numerical calculations and available experimental data. A modified electronic distribution function has been developed to simulate the charge transfer associated with compound formation. The effects of such a "charge transfer" modification to the embedded atom method are evaluated. The results of these simulations indicate that the embedded atom method combined with molecular dynamics may be used as a method to predict reasonably the transport properties.
Time-Dependent Molecular Reaction Dynamics
NASA Astrophysics Data System (ADS)
Öhrn, Yngve
2007-11-01
This paper is a brief review of a time-dependent, direct, nonadiabatic theory of molecular processes called Electron Nuclear Dynamics (END). This approach to the study of molecular reaction dynamics is a hierarchical theory that can be applied at various levels of approximation. The simplest level of END uses classical nuclei and represents all electrons by a single, complex, determinantal wave function. The wave function parameters such as average nuclear positions and momenta, and molecular orbital coefcients carry the time dependence and serve as dynamical variables. Examples of application are given of the simplest level of END to ion-atom and ion-molecule reactions.
Hunt, Rosemary A R; Ludlow, R Frederick; Otto, Sijbren
2009-11-19
Multicomponent chemical systems that exhibit a network of covalent and intermolecular interactions may produce interesting and often unexpected chemical or physical behavior. The formation of aggregates is a well-recognized example and presents a particular analytical challenge. We now report the development of a numerical fitting method capable of estimating equilibrium constants for the formation of aggregates from the product distribution of a dynamic combinatorial library containing self-recognizing library members.
Lin, Yu; Tan, Yeqiang; Qiu, Biwei; Shangguan, Yonggang; Harkin-Jones, Eileen; Zheng, Qiang
2013-01-17
The influence of annealing above the glass transition temperature (T(g)) on chain entanglement and molecular dynamics of solution-cast poly(methyl methacrylate)/poly(styrene-co-maleic anhydride) (PMMA/SMA) blends was investigated via a combination of dynamic rheological measurement and broadband dielectric spectroscopy. Chain entanglement density increases when the annealing temperature and/or time increases, resulting from the increased efficiency of chain packing and entanglement recovery. The results of the annealing treatment without cooling revealed that the increase of the entanglement density occurred during the annealing process instead of the subsequent cooling procedure. Annealing above T(g) exerts a profound effect on segmental motion, including the transition temperature and dynamics. Namely, T(g) shifts to higher temperatures and the relaxation time (τ(max)) increases due to the increased entanglement density and decreased molecular mobility. Either T(g) or τ(max) approaches an equilibrium value gradually, corresponding to the equilibrium entanglement density that might be obtained through the theoretical predictions. However, no obvious distribution broadening is observed due to the unchanged heterogeneous dynamics. Furthermore, side group rotational motion could be freely achieved without overcoming the chain entanglement resistance. Hence, neither the dynamics nor the distribution width of the subglass relaxation (β- and γ-relaxation) processes is affected by chain entanglement resulting from annealing, indicating that the local environment of the segments is unchanged.
Influence of solid-liquid interactions on dynamic wetting: a molecular dynamics study.
Bertrand, Emilie; Blake, Terence D; Coninck, Joël De
2009-11-18
Large-scale molecular dynamics (MD) simulations of liquid drops spreading on a solid substrate have been carried out for a very wide range of solid-liquid interactions and equilibrium contact angles. The results for these systems are shown to be consistent with the molecular-kinetic theory (MKT) of dynamic wetting, which emphasizes the role of contact-line friction as the principal channel of energy dissipation. Several predictions have been confirmed. These include a quantitative link between the dynamics of wetting and the work of adhesion and the existence of an optimum equilibrium contact angle that maximizes the speed of wetting. A feature of the new work is that key parameters (κ(0) and λ), normally accessible only by fitting the MKT to dynamic contact angle data, are also obtained directly from the simulations, with good agreement between the two sources. This validates the MKT at some fundamental level. Further verification is provided by contact angle relaxation studies, which also lend support to the interfacial tension relaxation process invoked in Shikhmurzaev's hydrodynamic model of dynamic wetting.
Influence of solid-liquid interactions on dynamic wetting: a molecular dynamics study
NASA Astrophysics Data System (ADS)
Bertrand, Emilie; Blake, Terence D.; De Coninck, Joël
2009-11-01
Large-scale molecular dynamics (MD) simulations of liquid drops spreading on a solid substrate have been carried out for a very wide range of solid-liquid interactions and equilibrium contact angles. The results for these systems are shown to be consistent with the molecular-kinetic theory (MKT) of dynamic wetting, which emphasizes the role of contact-line friction as the principal channel of energy dissipation. Several predictions have been confirmed. These include a quantitative link between the dynamics of wetting and the work of adhesion and the existence of an optimum equilibrium contact angle that maximizes the speed of wetting. A feature of the new work is that key parameters (κ0 and λ), normally accessible only by fitting the MKT to dynamic contact angle data, are also obtained directly from the simulations, with good agreement between the two sources. This validates the MKT at some fundamental level. Further verification is provided by contact angle relaxation studies, which also lend support to the interfacial tension relaxation process invoked in Shikhmurzaev's hydrodynamic model of dynamic wetting.
Hu, Xiaohu; Hong, Liang; Smith, Micholas Dean; Neusius, Thomas; Cheng, Xiaolin; Smith, Jeremy C.
2015-11-23
Here, internal motions of proteins are essential to their function. The time dependence of protein structural fluctuations is highly complex, manifesting subdiffusive, non-exponential behavior with effective relaxation times existing over many decades in time, from ps up to ~10^{2}s (refs 1-4). Here, using molecular dynamics simulations, we show that, on timescales from 10^{–12} to 10^{–5}s, motions in single proteins are self-similar, non-equilibrium and exhibit ageing. The characteristic relaxation time for a distance fluctuation, such as inter-domain motion, is observation-time-dependent, increasing in a simple, power-law fashion, arising from the fractal nature of the topology and geometry of the energy landscape explored. Diffusion over the energy landscape follows a non-ergodic continuous time random walk. Comparison with single-molecule experiments suggests that the non-equilibrium self-similar dynamical behavior persists up to timescales approaching the in vivo lifespan of individual protein molecules.
Hu, Xiaohu; Hong, Liang; Smith, Micholas Dean; ...
2015-11-23
Here, internal motions of proteins are essential to their function. The time dependence of protein structural fluctuations is highly complex, manifesting subdiffusive, non-exponential behavior with effective relaxation times existing over many decades in time, from ps up to ~102s (refs 1-4). Here, using molecular dynamics simulations, we show that, on timescales from 10–12 to 10–5s, motions in single proteins are self-similar, non-equilibrium and exhibit ageing. The characteristic relaxation time for a distance fluctuation, such as inter-domain motion, is observation-time-dependent, increasing in a simple, power-law fashion, arising from the fractal nature of the topology and geometry of the energy landscape explored.more » Diffusion over the energy landscape follows a non-ergodic continuous time random walk. Comparison with single-molecule experiments suggests that the non-equilibrium self-similar dynamical behavior persists up to timescales approaching the in vivo lifespan of individual protein molecules.« less
Molecular dynamics studies of interfacial water at the alumina surface.
Argyris, Dr. Dimitrios; Ho, Thomas; Cole, David
2011-01-01
Interfacial water properties at the alumina surface were investigated via all-atom equilibrium molecular dynamics simulations at ambient temperature. Al-terminated and OH-terminated alumina surfaces were considered to assess the structural and dynamic behavior of the first few hydration layers in contact with the substrates. Density profiles suggest water layering up to {approx}10 {angstrom} from the solid substrate. Planar density distribution data indicate that water molecules in the first interfacial layer are organized in well-defined patterns dictated by the atomic terminations of the alumina surface. Interfacial water exhibits preferential orientation and delayed dynamics compared to bulk water. Water exhibits bulk-like behavior at distances greater than {approx}10 {angstrom} from the substrate. The formation of an extended hydrogen bond network within the first few hydration layers illustrates the significance of water?water interactions on the structural properties at the interface.
Alt, Tobias; Knicker, Axel J; Strüder, Heiko K
2017-04-01
Analytical methods to assess thigh muscle balance need to provide reliable data to allow meaningful interpretation. However, reproducibility of the dynamic control ratio at the equilibrium point has not been evaluated yet. Therefore, the aim of this study was to compare relative and absolute reliability indices of its angle and moment values with conventional and functional hamstring-quadriceps ratios. Furthermore, effects of familiarisation and angular velocity on reproducibility were analysed. A number of 33 male volunteers participated in 3 identical test sessions. Peak moments (PMs) were determined unilaterally during maximum concentric and eccentric knee flexion (prone) and extension (supine position) at 0.53, 1.57 and 2.62 rad · s(-1). A repeated measure, ANOVA, confirmed systematic bias. Intra-class correlation coefficients and standard errors of measurement indicated relative and absolute reliability. Correlation coefficients were averaged over respective factors and tested for significant differences. All balance scores showed comparable low-to-moderate relative (<0.8-0.9) and good absolute reliability (<10%). Relative reproducibility of dynamic control equilibrium parameters augmented with increasing angular velocity, but not with familiarisation. At 2.62 rad · s(-1), high (moment: 0.906) to moderate (angle: 0.833) relative reliability scores with accordingly high absolute indices (4.9% and 6.4%) became apparent. Thus, the dynamic control equilibrium is an equivalent method for the reliable assessment of thigh muscle balance.
Hele, Timothy J. H.; Willatt, Michael J.; Muolo, Andrea; Althorpe, Stuart C.
2015-05-21
We recently obtained a quantum-Boltzmann-conserving classical dynamics by making a single change to the derivation of the “Classical Wigner” approximation. Here, we show that the further approximation of this “Matsubara dynamics” gives rise to two popular heuristic methods for treating quantum Boltzmann time-correlation functions: centroid molecular dynamics (CMD) and ring-polymer molecular dynamics (RPMD). We show that CMD is a mean-field approximation to Matsubara dynamics, obtained by discarding (classical) fluctuations around the centroid, and that RPMD is the result of discarding a term in the Matsubara Liouvillian which shifts the frequencies of these fluctuations. These findings are consistent with previous numerical results and give explicit formulae for the terms that CMD and RPMD leave out.
Dynamics and Feedback Control of Plasma Equilibrium Position in a Tokamak.
NASA Astrophysics Data System (ADS)
Burenko, Oleg
A brief history of the beginnings of nuclear fusion research involving toroidal closed-system magnetic plasma containment is presented. A tokamak machine is defined mathematically for the purposes of plasma equilibrium position perturbation analysis. The perturbation equations of a tokamak plasma equilibrium position are developed. Solution of the approximated perturbation equations is carried out. A unique, simple, and useful plasma displacement dynamics transfer function of a tokamak is developed. The dominant time constants of the dynamics transfer function are determined in a symbolic form. This symbolic form of the dynamics transfer function makes it possible to study the stability of a tokamak's plasma equilibrium position. Knowledge of the dynamics transfer function permits systematic syntheses of the required plasma displacement feedback control systems. The major parameters governing the plasma equilibrium position stability of a tokamak are shown to be (1) external magnetic field decay index, (2) transformer iron core effect, (3) plasma current, (4) radial rate-of-change inductance parameter, (5) vertical rate-of-change inductance parameter, and (6) vacuum vessel eddy-current time constant. An important and unique result is derived, showing that for a vacuum vessel eddy-current time constant exceeding a certain value the vertical plasma equilibrium position is stable, in spite of an intentional vertical instability design represented by a negative decay index. It is shown that a tokamak design having a theoretical set of positive decay index, negative radical rate-of-change inductance parameter, and positive vertical rate-of-change inductance parameter is expected to have a better plasma equilibrium position stability tolerance than a tokamak design having the same set with the signs reversed. The results of an actual hardware ISX-A tokamak plasma displacement feed-back control system design are presented. It is shown that a theoretical design computer
Chemistry and Chemical Equilibrium Dynamics of BMAA and Its Carbamate Adducts.
Diaz-Parga, Pedro; Goto, Joy J; Krishnan, V V
2017-09-18
Beta-N-methylamino-L-alanine (BMAA) has been demonstrated to contribute to the onset of the ALS/Parkinsonism-dementia complex (ALS/PDC) and is implicated in the progression of other neurodegenerative diseases. While the role of BMAA in these diseases is still debated, one of the suggested mechanisms involves the activation of excitatory glutamate receptors. In particular, the excitatory effects of BMAA are shown to be dependent on the presence of bicarbonate ions, which in turn forms carbamate adducts in physiological conditions. The formation of carbamate adducts from BMAA and bicarbonate is similar to the formation of carbamate adducts from non-proteinogenic amino acids. Structural, chemical, and biological information related to non-proteinogenic amino acids provide insight into the formation of and possible neurological action of BMAA. This article reviews the carbamate formation of BMAA in the presence of bicarbonate ions, with a particular focus on how the chemical equilibrium of BMAA carbamate adducts may affect the molecular mechanism of its function. Highlights of nuclear magnetic resonance (NMR)-based studies on the equilibrium process between free BMAA and its adducts are presented. The role of divalent metals on the equilibrium process is also explored. The formation and the equilibrium process of carbamate adducts of BMAA may answer questions on their neuroactive potency and provide strong motivation for further investigations into other toxic mechanisms.
Out-of-equilibrium dynamics of coherent non-Abelian gauge fields
NASA Astrophysics Data System (ADS)
Berges, Jürgen; Scheffler, Sebastian; Schlichting, Sören; Sexty, Dénes
2012-02-01
We study out-of-equilibrium dynamics of intense non-Abelian gauge fields. Generalizing the well-known Nielsen-Olesen instabilities for constant initial color-magnetic fields, we investigate the impact of temporal modulations and fluctuations in the initial conditions. This leads to a remarkable coexistence of the original Nielsen-Olesen instability and the subdominant phenomenon of parametric resonance. Taking into account that the fields may be correlated only over a limited transverse size, we model characteristic aspects of the dynamics of color flux tubes relevant in the context of heavy-ion collisions.
Dynamical passage to approximate equilibrium shapes for spinning, gravitating rubble asteroids
NASA Astrophysics Data System (ADS)
Sharma, Ishan; Jenkins, James T.; Burns, Joseph A.
2009-03-01
Many asteroids are thought to be particle aggregates held together principally by self-gravity. Here we study — for static and dynamical situations — the equilibrium shapes of spinning asteroids that are permitted for rubble piles. As in the case of spinning fluid masses, not all shapes are compatible with a granular rheology. We take the asteroid to always be an ellipsoid with an interior modeled as a rigid-plastic, cohesion-less material with a Drucker-Prager yield criterion. Using an approximate volume-averaged procedure, based on the classical method of moments, we investigate the dynamical process by which such objects may achieve equilibrium. We first collapse our dynamical approach to its statical limit to derive regions in spin-shape parameter space that allow equilibrium solutions to exist. At present, only a graphical illustration of these solutions for a prolate ellipsoid following the Drucker-Prager failure law is available [Sharma, I., Jenkins, J.T., Burns, J.A., 2005a. Bull. Am. Astron. Soc. 37, 643; Sharma, I., Jenkins, J.T., Burns, J.A., 2005b. Equilibrium shapes of ellipsoidal soil asteroids. In: García-Rojo, R., Hermann, H.J., McNamara, S. (Eds.), Proceedings of the 5th International Conference on Micromechanics of Granular Media, vol. 1. A.A. Balkema, UK; Holsapple, K.A., 2007. Icarus 187, 500-509]. Here, we obtain the equilibrium landscapes for general triaxial ellipsoids, as well as provide the requisite governing formulae. In addition, we demonstrate that it may be possible to better interpret the results of Richardson et al. [Richardson, D.C., Elankumaran, P., Sanderson, R.E., 2005. Icarus 173, 349-361] within the context of a Drucker-Prager material. The graphical result for prolate ellipsoids in the static limit is the same as those of Holsapple [Holsapple, K.A., 2007. Icarus 187, 500-509] because, when worked out, his final equations will match ours. This is because, though the formalisms to reach these expressions differ, in statics
Equilibrium and dynamic study on hexavalent chromium adsorption onto activated carbon.
Di Natale, F; Erto, A; Lancia, A; Musmarra, D
2015-01-08
In this work, the results of equilibrium and dynamic adsorption tests of hexavalent chromium, Cr (VI), on activated carbon are presented. Adsorption isotherms were determined at different levels of pH and temperature. Dynamic tests were carried out in terms of breakthrough curves of lab-scale fixed bed column at different pH, inlet concentration and flow rate. Both the adsorption isotherms and the breakthrough curves showed non-linear and unconventional trends. The experimental results revealed that chromium speciation played a key role in the adsorption process, also for the occurrence of Cr(VI)-to-Cr(III) reduction reactions. Equilibrium tests were interpreted in light of a multi-component Langmuir model supported by ion speciation analysis. For the interpretation of the adsorption dynamic tests, a mass transfer model was proposed. Dynamic tests at pH 11 were well described considering the external mass transfer as the rate controlling step. Differently, for dynamic tests at pH 6 the same model provided a satisfying description of the experimental breakthrough curves only until a sorbent coverage around 1.6mgg(-1). Above this level, a marked reduction of the breakthrough curve slope was observed in response to a transition to an inter-particle adsorption mechanism.
Probing the out-of-equilibrium dynamics of two interacting atoms
NASA Astrophysics Data System (ADS)
Keller, Tim; Fogarty, Thomás
2016-12-01
We study the out-of-equilibrium dynamics of two interacting atoms in a one-dimensional harmonic trap after a quench by a tightly pinned impurity atom. We make use of an approximate variational calculation called the Lagrange-mesh method to solve the Schrödinger equation as a function of interparticle interaction and impurity quench strength. We investigate the out-of-equilibrium dynamics by calculating the Loschmidt echo which quantifies the irreversibility of the system following the quench, while its probability distribution after long times can be used to identify distinct dynamical regimes. These quantities are related to the spectral function which describes the full dynamical spectrum, and we show through a thorough examination of the parameter space the existence of distinct scattering states and collective oscillations. This work demonstrates how these dynamics are strongly dependent on the interaction strength between the atoms and may be tuned to observe the establishment of the orthogonality catastrophe in few-body systems.
Molecular dynamics on hypercube parallel computers
NASA Astrophysics Data System (ADS)
Smith, W.
1991-03-01
The implementation of molecular dynamics on parallel computers is described, with particular reference to hypercube computers. Three particular algorithms are described: replicated data (RD); systolic loop (SLS-G), and parallelised link-cells (PLC), all of which have good load balancing. The performance characteristics of each algorithm and the factors affecting their scaling properties are discussed. The article is pedagogic in intent, to introduce a novice to the main aspects of parallel computing in molecular dynamics.
Toward a molecular theory of homogeneous bubble nucleation: I. Equilibrium embryo definition.
Torabi, Korosh; Corti, David S
2013-10-17
We propose an equilibrium embryo definition for homogeneous bubble nucleation within the pure component superheated Lennard-Jones liquid. The suggested embryo definition serves as an improvement to a previous (n,v) embryo model (Uline and Corti 2007). In that model, the constrained equilibrium between the bubble and the surrounding superheated liquid was maintained by placing n particles within a spherical volume v, without concern for the redundant counting of configurations and the relevance of the model to the dynamics of a nucleation process. To eliminate this redundancy, while only considering those embryos that should most likely appear at a transitional state, we now define the volume of the embryo via the use of a shell particle and only include n particles inside the volume that are deemed to be "vapor-like". The underlying free energy surface of formation of the new (n,v) embryo model is generated via Monte Carlo simulation and also approximately by a suggested density functional theory method. The resulting surface implies that small and locally confined regions of near-zero density serve as precursors initiating homogeneous bubble nucleation. Furthermore, the nonredundant counting of the embryo configurations yields a well-defined and sharp conduit in the free energy surface that directs the initially formed embryos toward the critical nucleus. We discuss how the suggested equilibrium embryo model aids in both the identification of the transitional configurations and calculation of the average number density of the critical nuclei within the superheated liquid phase, which is the focus of the companion paper (DOI 10.1021/jp404151h).
Bjorgaard, J A; Velizhanin, K A; Tretiak, S
2016-04-21
The effects of solvent on molecular processes such as excited state relaxation and photochemical reaction often occurs in a nonequilibrium regime. Dynamic processes such as these can be simulated using excited statemolecular dynamics. In this work, we describe methods of simulating nonequilibrium solvent effects in excited statemolecular dynamics using linear-response time-dependent density functional theory and apparent surface charge methods. These developments include a propagation method for solvent degrees of freedom and analytical energy gradients for the calculation of forces. Molecular dynamics of acetaldehyde in water or acetonitrile are demonstrated where the solute-solvent system is out of equilibrium due to photoexcitation and emission.
Modeling the Hydrogen Bond within Molecular Dynamics
ERIC Educational Resources Information Center
Lykos, Peter
2004-01-01
The structure of a hydrogen bond is elucidated within the framework of molecular dynamics based on the model of Rahman and Stillinger (R-S) liquid water treatment. Thus, undergraduates are exposed to the powerful but simple use of classical mechanics to solid objects from a molecular viewpoint.
Molecular ions, Rydberg spectroscopy and dynamics
Jungen, Ch.
2015-01-22
Ion spectroscopy, Rydberg spectroscopy and molecular dynamics are closely related subjects. Multichannel quantum defect theory is a theoretical approach which draws on this close relationship and thereby becomes a powerful tool for the study of systems consisting of a positively charged molecular ion core interacting with an electron which may be loosely bound or freely scattering.
Molecular Dynamics Simulations of Simple Liquids
ERIC Educational Resources Information Center
Speer, Owner F.; Wengerter, Brian C.; Taylor, Ramona S.
2004-01-01
An experiment, in which students were given the opportunity to perform molecular dynamics simulations on a series of molecular liquids using the Amber suite of programs, is presented. They were introduced to both physical theories underlying classical mechanics simulations and to the atom-atom pair distribution function.
Modeling the Hydrogen Bond within Molecular Dynamics
ERIC Educational Resources Information Center
Lykos, Peter
2004-01-01
The structure of a hydrogen bond is elucidated within the framework of molecular dynamics based on the model of Rahman and Stillinger (R-S) liquid water treatment. Thus, undergraduates are exposed to the powerful but simple use of classical mechanics to solid objects from a molecular viewpoint.
Molecular Dynamics Simulations of Simple Liquids
ERIC Educational Resources Information Center
Speer, Owner F.; Wengerter, Brian C.; Taylor, Ramona S.
2004-01-01
An experiment, in which students were given the opportunity to perform molecular dynamics simulations on a series of molecular liquids using the Amber suite of programs, is presented. They were introduced to both physical theories underlying classical mechanics simulations and to the atom-atom pair distribution function.
Tassis, Konstantinos; Willacy, Karen; Yorke, Harold W.; Turner, Neal J.
2012-07-01
We combine dynamical and non-equilibrium chemical modeling of evolving prestellar molecular cloud cores and investigate the evolution of molecular abundances in the contracting core. We model both magnetic cores, with varying degrees of initial magnetic support, and non-magnetic cores, with varying collapse delay times. We explore, through a parameter study, the competing effects of various model parameters in the evolving molecular abundances, including the elemental C/O ratio, the temperature, and the cosmic-ray ionization rate. We find that different models show their largest quantitative differences at the center of the core, whereas the outer layers, which evolve slower, have abundances which are severely degenerate among different dynamical models. There is a large range of possible abundance values for different models at a fixed evolutionary stage (central density), which demonstrates the large potential of chemical differentiation in prestellar cores. However, degeneracies among different models, compounded with uncertainties induced by other model parameters, make it difficult to discriminate among dynamical models. To address these difficulties, we identify abundance ratios between particular molecules, the measurement of which would have maximal potential for discrimination among the different models examined here. In particular, we find that the ratios between NH{sub 3} and CO, NH{sub 2} and CO, and NH{sub 3} and HCO{sup +} are sensitive to the evolutionary timescale, and that the ratio between HCN and OH is sensitive to the C/O ratio. Finally, we demonstrate that measurements of the central deviation (central depletion or enhancement) of abundances of certain molecules are good indicators of the dynamics of the core.
Shi, Ze; Castro, Carlos E; Arya, Gaurav
2017-05-23
Structural DNA nanotechnology, the assembly of rigid 3D structures of complex yet precise geometries, has recently been used to design dynamic, mechanically compliant nanostructures with tunable equilibrium conformations and conformational distributions. Here we use coarse-grained molecular dynamics simulations to provide insights into the conformational dynamics of a set of mechanically compliant DNA nanostructures-DNA hinges that use single-stranded DNA "springs" to tune the equilibrium conformation of a layered double-stranded DNA "joint" connecting two stiff "arms" constructed from DNA helix bundles. The simulations reproduce the experimentally measured equilibrium angles between hinge arms for a range of hinge designs. The hinges are found to be structurally stable, except for some fraying of the open ends of the DNA helices comprising the hinge arms and some loss of base-pairing interactions in the joint regions coinciding with the crossover junctions, especially in hinges designed to exhibit a small bending angle that exhibit large local stresses resulting in strong kinks in their joints. Principal component analysis reveals that while the hinge dynamics are dominated by bending motion, some twisting and sliding of hinge arms relative to each other also exists. Forced deformation of the hinges reveals distinct bending mechanisms for hinges with short, inextensible springs versus those with longer, more extensible springs. Lastly, we introduce an approach for rapidly predicting equilibrium hinge angles from individual force-deformation behaviors of its single- and double-stranded DNA components. Taken together, these results demonstrate that coarse-grained modeling is a promising approach for designing, predicting, and studying the dynamics of compliant DNA nanostructures, where conformational fluctuations become important, multiple deformation mechanisms exist, and continuum approaches may not yield accurate properties.
Fermionic Molecular Dynamics for Nuclear Dynamics and Thermodynamics
NASA Astrophysics Data System (ADS)
Hasnaoui, K. H. O.; Chomaz, Ph; Gulminelli, F.
A new Fermionic Molecular Dynamics (FMD) model based on a Skyrme functional is proposed in this paper. After introducing the basic formalism, some first applications to nuclear structure and nuclear thermodynamics are presented.
Wu, Wei; Wang, Jin
2014-09-14
We have established a general non-equilibrium thermodynamic formalism consistently applicable to both spatially homogeneous and, more importantly, spatially inhomogeneous systems, governed by the Langevin and Fokker-Planck stochastic dynamics with multiple state transition mechanisms, using the potential-flux landscape framework as a bridge connecting stochastic dynamics with non-equilibrium thermodynamics. A set of non-equilibrium thermodynamic equations, quantifying the relations of the non-equilibrium entropy, entropy flow, entropy production, and other thermodynamic quantities, together with their specific expressions, is constructed from a set of dynamical decomposition equations associated with the potential-flux landscape framework. The flux velocity plays a pivotal role on both the dynamic and thermodynamic levels. On the dynamic level, it represents a dynamic force breaking detailed balance, entailing the dynamical decomposition equations. On the thermodynamic level, it represents a thermodynamic force generating entropy production, manifested in the non-equilibrium thermodynamic equations. The Ornstein-Uhlenbeck process and more specific examples, the spatial stochastic neuronal model, in particular, are studied to test and illustrate the general theory. This theoretical framework is particularly suitable to study the non-equilibrium (thermo)dynamics of spatially inhomogeneous systems abundant in nature. This paper is the second of a series.
Wu, Wei; Wang, Jin
2014-09-14
We have established a general non-equilibrium thermodynamic formalism consistently applicable to both spatially homogeneous and, more importantly, spatially inhomogeneous systems, governed by the Langevin and Fokker-Planck stochastic dynamics with multiple state transition mechanisms, using the potential-flux landscape framework as a bridge connecting stochastic dynamics with non-equilibrium thermodynamics. A set of non-equilibrium thermodynamic equations, quantifying the relations of the non-equilibrium entropy, entropy flow, entropy production, and other thermodynamic quantities, together with their specific expressions, is constructed from a set of dynamical decomposition equations associated with the potential-flux landscape framework. The flux velocity plays a pivotal role on both the dynamic and thermodynamic levels. On the dynamic level, it represents a dynamic force breaking detailed balance, entailing the dynamical decomposition equations. On the thermodynamic level, it represents a thermodynamic force generating entropy production, manifested in the non-equilibrium thermodynamic equations. The Ornstein-Uhlenbeck process and more specific examples, the spatial stochastic neuronal model, in particular, are studied to test and illustrate the general theory. This theoretical framework is particularly suitable to study the non-equilibrium (thermo)dynamics of spatially inhomogeneous systems abundant in nature. This paper is the second of a series.
Scaling of Langevin and molecular dynamics persistence times of nonhomogeneous fluids.
Olivares-Rivas, Wilmer; Colmenares, Pedro J
2012-01-01
The existing solution for the Langevin equation of an anisotropic fluid allowed the evaluation of the position-dependent perpendicular and parallel diffusion coefficients, using molecular dynamics data. However, the time scale of the Langevin dynamics and molecular dynamics are different and an ansatz for the persistence probability relaxation time was needed. Here we show how the solution for the average persistence probability obtained from the backward Smoluchowski-Fokker-Planck equation (SE), associated to the Langevin dynamics, scales with the corresponding molecular dynamics quantity. Our SE perpendicular persistence time is evaluated in terms of simple integrals over the equilibrium local density. When properly scaled by the perpendicular diffusion coefficient, it gives a good match with that obtained from molecular dynamics.
Non-equilibrium disordered Bose gases: condensation, superfluidity and dynamical Bose glass
NASA Astrophysics Data System (ADS)
Chen, Lei; Liang, Zhaoxin; Hu, Ying; Zhang, Zhidong
2016-01-01
In an equilibrium three-dimensional (3D) disordered condensate, it is well established that disorder can generate an amount of normal fluid ρ n equaling to 4/3 of ρ ex , where ρ ex is a sum of interaction-induced quantum depletion and disorder-induced condensate deformation. The concept that the superfluid is more volatile to the existence of disorder than the condensate is crucial to the understanding of the Bose glass phase. In this work, we show that, by bringing a weakly disordered 3D condensate to non-equilibrium regime via a quantum quench in the interaction, disorder can destroy superfluid significantly more, leading to a steady state of Hamiltonian H f in which the ρ n far exceeds 4/3 of the ρ ex . This suggests the possibility of engineering Bose glass in the dynamic regime. Here, we refer to the dynamical Bose glass as the case where in the steady state of quenched condensate, the superfluid density goes to zero while the condensate density remains finite. As both the ρ n and ρ ex are measurable quantities, our results allow an experimental demonstration of the dramatized interplay between the disorder and interaction in the non-equilibrium scenario.
A Hierarchy of Dynamic Equilibria and a View of a Fly's Equilibrium Reflex
NASA Astrophysics Data System (ADS)
Wang, Z. Jane
Understanding structures within a structure is a topic that has fascinated Leo throughout his life, and we are now benefiting from his fundamental insights when we think about living organisms. A living organism is far from statistical equilibrium and it does not have a single critical parameter. Nevertheless, each organism has a hierarchical structure within itself. Recently, asking how often a fly must sense its orientation in order to balance in air has led us to suggest one of the fly's 17 steering muscles, the first basalar muscle, is responsible for maintaining flight stability. Here I suggest that the chain of events associated with flight equilibrium reflex can be viewed as a succession of local linear transformation about a set of dynamic equilibria. Each of the functionally different parts, the sensors, motor neurons, muscles, wing-hinges, flapping wings, and the thorax, operates near its own dynamic equilibrium, often close to the boundary between stability and instability. Locomotion rises as an organism responds to a small perturbation from these equilibria. Kadanoff session.
Collective non-equilibrium dynamics at surfaces and the spatio-temporal edge
NASA Astrophysics Data System (ADS)
Marcuzzi, M.; Gambassi, A.; Pleimling, M.
2012-11-01
Symmetries represent a fundamental constraint for physical systems and relevant new phenomena often emerge as a consequence of their breaking. An important example is provided by space- and time-translational invariance in statistical systems, which hold at a coarse-grained scale in equilibrium and are broken by spatial and temporal boundaries, the former being implemented by surfaces —unavoidable in real samples— the latter by some initial condition for the dynamics which causes a non-equilibrium evolution. While the separate effects of these two boundaries are well understood, we demonstrate here that additional, unexpected features arise upon approaching the effective edge formed by their intersection. For this purpose, we focus on the classical semi-infinite Ising model with spin-flip dynamics evolving out of equilibrium at its critical point. Considering both subcritical and critical values of the coupling among surface spins, we present numerical evidence of a scaling regime with universal features which emerges upon approaching the spatio-temporal edge and we rationalise these findings within a field-theoretical approach.
Molecular dynamics simulations of nanostructures
NASA Astrophysics Data System (ADS)
Yuan, Zaoshi
This dissertation is focused on multimillion-atom molecular dynamics (MD) simulations of nanoscale materials. In the past decade, nanoscale materials have made significant commercial impacts, which will potentially lead to the next industrial revolution. The interest lies in the novel and promising features nanoscale materials exhibit due to their confined sizes. However, not all novel behaviors are understood or controllable. Many uncontrollable parameters, e.g. defects and dangling bonds, are known to hinder the performance of nanodevices. Solutions to these problems rely on our understanding of fundamental elements in nanoscience: isolated individual nanostructures and their assemblies. In this dissertation, we will address atomistic foundations of several problems of technological importance in nanoscience. Specifically, three basic problems are discussed: (1) embrittlement of nanocrystalline metal; (2) novel thermo-mechanical behaviors of nanowires (NWs); and (3) planar defect generation in NWs. With a scalable algorithm implemented on massively parallel computing platforms and various data mining methods, MD simulations can provide valuable insights into these problems. An essential role of sulfur segregation-induced amorphization of crystalline nickel was recently discovered experimentally, but the atomistic mechanism of the amorphization remains unexplained. Our MD simulations reveal that the large steric size of sulfur impurity causes strong sulfur-sulfur interaction mediated by lattice distortion, which leads to amorphization near the percolation threshold at the sulfur-sulfur network in nickel crystal. The generality of the mechanism due to the percolation of an impurity network is further confirmed by a model binary system. In our study of novel behaviors of semiconductor NWs, MD simulations construct a rich size-temperature `phase diagram' for the mechanical response of a zinc-oxide NW under tension. For smaller diameters and higher temperatures, novel
On Mechanical Properties of Graphene Sheet Estimated Using Molecular Dynamics Simulations
NASA Astrophysics Data System (ADS)
Das, D. K.; Ghosh, M. M.
2017-09-01
This work reports estimation of mechanical properties, particularly Young's modulus of a single-layered graphene sheet by molecular dynamics (MD) simulation-based four different approaches, viz. tensile modeling, bending modeling, oscillation modeling and equilibrium MD modeling. The Young's modulus is estimated to be of the order of some TPa. The equilibrium MD method has yielded a Young's modulus value lower than the other non-equilibrium methods, due to the absence of any external forcing factor. Among the non-equilibrium MD methods, the bending modeling is found to predict the highest value of Young's modulus. Comparison among different non-equilibrium methods has established the effect of strain rate on the estimated value of the Young's modulus. The MD simulation-based approaches adopted here can be useful for the design of graphene and graphene-based materials in advanced mechanical applications.
NASA Astrophysics Data System (ADS)
Capuzzi, P.; Hernández, E. S.; Szybisz, L.
2008-10-01
We present a generalization of the fluid-dynamical scheme developed for nuclear physics to the case of two trapped fermion species with pairing interactions. To establish a macroscopic description of the mass and momentum conservation laws, we adopt a generalization of the usual Thomas-Fermi approach that includes the pairing energy. We analyze the equilibrium density and gap profiles for an equal population mixture of harmonically trapped Li6 atoms for different choices of the local equation of state. We examine slight departures from equilibrium within our formulation, finding that density oscillations can propagate as first sound coupled to pairing vibrations, that in a homogeneous fermion system exhibit a Bogoliubov-like quasiparticle spectrum. In this case, the dispersion relation for the coupled modes displays a rich scenario of stable, unstable, and damped regimes.
Far from equilibrium dynamics of Bose-Einstein condensation for axion dark matter
NASA Astrophysics Data System (ADS)
Berges, Jürgen; Jaeckel, Joerg
2015-01-01
Axions and similar very weakly interacting particles are increasingly compelling candidates for the cold dark matter of the Universe. Having very low mass and being produced nonthermally in the early universe, axions feature extremely high occupation numbers. It has been suggested that this leads to the formation of a Bose-Einstein condensate with potentially significant impact on observation and direct detection experiments. In this paper we aim to clarify that if Bose-Einstein condensation occurs for light and very weakly interacting dark matter particles, it does not happen in thermal equilibrium but is described by a far from equilibrium state. In particular we point out that the dynamics is characterized by two very different time scales, such that condensation occurs on a much shorter time scale than full thermalization.
NASA Astrophysics Data System (ADS)
Oliva, L.; Ruggieri, M.; Plumari, S.; Scardina, F.; Greco, V.
2017-04-01
In this study we discuss our results on the spectrum of photons emitted from the quark-gluon plasma produced in heavy ion collisions at RHIC energies. Simulating the space-time evolution of the fireball by solving the relativistic Boltzmann transport equation and including two-particle scattering processes with photon emission allows us to make a first step in the description of thermal photons from the QGP as well as of those produced in the pre-equilibrium stage. Indeed, we consider not only a standard Glauber initial condition but also a model in which quarks and gluons are produced in the very early stage through the Schwinger mechanism by the decay of an initial color-electric field. In the latter approach relativistic kinetic equations are coupled in a self-consistent way to field equations. We aim at spotting the impact of early stage non-equilibrium dynamics on the photon production.
Out-of-equilibrium charge dynamics in a hybrid circuit quantum electrodynamics architecture
NASA Astrophysics Data System (ADS)
Viennot, J. J.; Delbecq, M. R.; Dartiailh, M. C.; Cottet, A.; Kontos, T.
2014-04-01
The recent development of hybrid circuit quantum electrodynamics allows one to study how cavity photons interact with a system driven out of equilibrium by fermionic reservoirs. We study here one of the simplest combination: a double quantum dot coupled to a single mode of the electromagnetic field. We are able to couple resonantly the charge levels of a carbon-nanotube-based double dot to cavity photons. We perform a microwave readout of the charge states of this system, which allows us to unveil features of the out-of-equilibrium charge dynamics, otherwise invisible in the DC current. We extract the relaxation rate, dephasing rate, and photon number of the hybrid system using a theory based on a master equation technique. These findings open the path for manipulating other degrees of freedom, e.g., the spin and/or the valley in nanotube-based double dots using microwave light.
Sand ripple dynamics in the case of out-of-equilibrium aeolian regimes.
Misbah, C; Valance, A
2003-12-01
From a phenomenological hydrodynamical model, we analyze the aeolian sand ripple evolution in an out-of-equilibrium aeolian regime where erosion exceeds accretion (and vice versa). We find, in particular, that the ripple structure can be destroyed in favor of a flat sand bed. In the ripple regime we report on a new class of generic dynamics described by the Benney equation. This equation reveals either order or disorder depending on whether wave dispersion is strong or weak. In both cases, the average wavelength of the pattern is fixed in time. This markedly contrasts with the regime of equilibrium aeolian regime -reached when erosion balances deposition- where ripples undergo a coarsening process at long time (i.e., the wavelength increases indefinitely with time).
Universal time fluctuations in near-critical out-of-equilibrium quantum dynamics.
Campos Venuti, Lorenzo; Zanardi, Paolo
2014-02-01
Out-of-equilibrium quantum systems display complex temporal patterns. Such time fluctuations are generically exponentially small in the system volume and therefore can be safely ignored in most of the cases. However, if one consider small quench experiments, time fluctuations can be greatly enhanced. We show that time fluctuations may become stronger than other forms of equilibrium quantum fluctuations if the quench is performed close to a critical point. For sufficiently relevant operators the full distribution function of dynamically evolving observable expectation values becomes a universal function uniquely characterized by the critical exponents and the boundary conditions. At regular points of the phase diagram and for nonsufficiently relevant operators the distribution becomes Gaussian. Our predictions are confirmed by an explicit calculation on the quantum Ising model.
Modeling hybrid perovskites by molecular dynamics.
Mattoni, Alessandro; Filippetti, Alessio; Caddeo, Claudia
2017-02-01
The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics.
Modeling hybrid perovskites by molecular dynamics
NASA Astrophysics Data System (ADS)
Mattoni, Alessandro; Filippetti, Alessio; Caddeo, Claudia
2017-02-01
The topical review describes the recent progress in the modeling of hybrid perovskites by molecular dynamics simulations. Hybrid perovskites and in particular methylammonium lead halide (MAPI) have a tremendous technological relevance representing the fastest-advancing solar material to date. They also represent the paradigm of an organic-inorganic crystalline material with some conceptual peculiarities: an inorganic semiconductor for what concerns the electronic and absorption properties with a hybrid and solution processable organic-inorganic body. After briefly explaining the basic concepts of ab initio and classical molecular dynamics, the model potential recently developed for hybrid perovskites is described together with its physical motivation as a simple ionic model able to reproduce the main dynamical properties of the material. Advantages and limits of the two strategies (either ab initio or classical) are discussed in comparison with the time and length scales (from pico to microsecond scale) necessary to comprehensively study the relevant properties of hybrid perovskites from molecular reorientations to electrocaloric effects. The state-of-the-art of the molecular dynamics modeling of hybrid perovskites is reviewed by focusing on a selection of showcase applications of methylammonium lead halide: molecular cations disorder; temperature evolution of vibrations; thermally activated defects diffusion; thermal transport. We finally discuss the perspectives in the modeling of hybrid perovskites by molecular dynamics.
Molecular dynamics simulations: advances and applications
Hospital, Adam; Goñi, Josep Ramon; Orozco, Modesto; Gelpí, Josep L
2015-01-01
Molecular dynamics simulations have evolved into a mature technique that can be used effectively to understand macromolecular structure-to-function relationships. Present simulation times are close to biologically relevant ones. Information gathered about the dynamic properties of macromolecules is rich enough to shift the usual paradigm of structural bioinformatics from studying single structures to analyze conformational ensembles. Here, we describe the foundations of molecular dynamics and the improvements made in the direction of getting such ensemble. Specific application of the technique to three main issues (allosteric regulation, docking, and structure refinement) is discussed. PMID:26604800
Molecular Dynamics Studies of Matrix Metalloproteases.
Díaz, Natalia; Suárez, Dimas
2017-01-01
Matrix metalloproteases are multidomain enzymes with a remarkable proteolytic activity located in the extracellular environment. Their catalytic activity and structural properties have been intensively studied during the last few decades using both experimental and theoretical approaches, but many open questions still remain. Extensive molecular dynamics simulations enable the sampling of the configurational space of a molecular system, thus contributing to the characterization of the structure, dynamics, and ligand binding properties of a particular MMP. Based on previous computational experience, we provide in this chapter technical and methodological guidelines that may be useful to and stimulate other researchers to perform molecular dynamics simulations to help address unresolved questions concerning the molecular mode of action of MMPs.
Kim, Y R; Hahn, J S; Hong, H; Jeong, W; Song, N W; Shin, H C; Kim, D
1999-01-11
The dynamic equilibrium unfolding pathway of human tumor necrosis factor-alpha (TNF-alpha) during denaturation at different guanidine hydrochloride (GdnHCl) concentrations (0-4.2 M) was investigated by steady-state fluorescence spectroscopy, potassium iodide (KI) fluorescence quenching, far-UV circular dichroism (CD), picosecond time-resolved fluorescence lifetime, and anisotropy decay measurements. We utilized the intrinsic fluorescence of Trp-28 and Trp-114 to characterize the conformational changes involved in the equilibrium unfolding pathway. The detailed unfolding pathway under equilibrium conditions was discussed with respect to motional dynamics and partially folded structures. At 0-0.9 M [GdnHCl], the rotational correlation times of 22-25 ns were obtained from fluorescence anisotropy decay measurements and assigned to those of trimeric states by hydrodynamic calculation. In this range, the solvent accessibility of Trp residues increased with increasing [GdnHCl], suggesting the slight expansion of the trimeric structure. At 1.2-2.1 M [GdnHCl], the enhanced solvent accessibility and the rotational degree of freedom of Trp residues were observed, implying the loosening of the internal structure. In this [GdnHCl] region, TNF-alpha was thought to be in soluble aggregates having distinct conformational characteristics from a native (N) or fully unfolded state (U). At 4.2 M [GdnHCl], TNF-alpha unfolded to a U-state. From these results, the equilibrium unfolding pathway of TNF-alpha, trimeric and all beta-sheet protein, could not be viewed from the simple two state model (N-->U).
Dynamic molecular crystals with switchable physical properties.
Sato, Osamu
2016-06-21
The development of molecular materials whose physical properties can be controlled by external stimuli - such as light, electric field, temperature, and pressure - has recently attracted much attention owing to their potential applications in molecular devices. There are a number of ways to alter the physical properties of crystalline materials. These include the modulation of the spin and redox states of the crystal's components, or the incorporation within the crystalline lattice of tunable molecules that exhibit stimuli-induced changes in their molecular structure. A switching behaviour can also be induced by changing the molecular orientation of the crystal's components, even in cases where the overall molecular structure is not affected. Controlling intermolecular interactions within a molecular material is also an effective tool to modulate its physical properties. This Review discusses recent advances in the development of such stimuli-responsive, switchable crystalline compounds - referred to here as dynamic molecular crystals - and suggests how different approaches can serve to prepare functional materials.
HTMD: High-Throughput Molecular Dynamics for Molecular Discovery.
Doerr, S; Harvey, M J; Noé, Frank; De Fabritiis, G
2016-04-12
Recent advances in molecular simulations have allowed scientists to investigate slower biological processes than ever before. Together with these advances came an explosion of data that has transformed a traditionally computing-bound into a data-bound problem. Here, we present HTMD, a programmable, extensible platform written in Python that aims to solve the data generation and analysis problem as well as increase reproducibility by providing a complete workspace for simulation-based discovery. So far, HTMD includes system building for CHARMM and AMBER force fields, projection methods, clustering, molecular simulation production, adaptive sampling, an Amazon cloud interface, Markov state models, and visualization. As a result, a single, short HTMD script can lead from a PDB structure to useful quantities such as relaxation time scales, equilibrium populations, metastable conformations, and kinetic rates. In this paper, we focus on the adaptive sampling and Markov state modeling features.
Phase-field investigation on the non-equilibrium interface dynamics of rapid alloy solidification
Choi, Jeong
2011-01-01
The research program reported here is focused on critical issues that represent conspicuous gaps in current understanding of rapid solidification, limiting our ability to predict and control microstructural evolution (i.e. morphological dynamics and microsegregation) at high undercooling, where conditions depart significantly from local equilibrium. More specifically, through careful application of phase-field modeling, using appropriate thin-interface and anti-trapping corrections and addressing important details such as transient effects and a velocity-dependent (i.e. adaptive) numerics, the current analysis provides a reasonable simulation-based picture of non-equilibrium solute partitioning and the corresponding oscillatory dynamics associated with single-phase rapid solidification and show that this method is a suitable means for a self-consistent simulation of transient behavior and operating point selection under rapid growth conditions. Moving beyond the limitations of conventional theoretical/analytical treatments of non-equilibrium solute partitioning, these results serve to substantiate recent experimental findings and analytical treatments for single-phase rapid solidification. The departure from the equilibrium solid concentration at the solid-liquid interface was often observed during rapid solidification, and the energetic associated non-equilibrium solute partitioning has been treated in detail, providing possible ranges of interface concentrations for a given growth condition. Use of these treatments for analytical description of specific single-phase dendritic and cellular operating point selection, however, requires a model for solute partitioning under a given set of growth conditions. Therefore, analytical solute trapping models which describe the chemical partitioning as a function of steady state interface velocities have been developed and widely utilized in most of the theoretical investigations of rapid solidification. However, these
High temperature phonon dispersion in graphene using classical molecular dynamics
Anees, P. Panigrahi, B. K.; Valsakumar, M. C.
2014-04-24
Phonon dispersion and phonon density of states of graphene are calculated using classical molecular dynamics simulations. In this method, the dynamical matrix is constructed based on linear response theory by computing the displacement of atoms during the simulations. The computed phonon dispersions show excellent agreement with experiments. The simulations are done in both NVT and NPT ensembles at 300 K and found that the LO/TO modes are getting hardened at the Γ point. The NPT ensemble simulations capture the anharmonicity of the crystal accurately and the hardening of LO/TO modes is more pronounced. We also found that at 300 K the C-C bond length reduces below the equilibrium value and the ZA bending mode frequency becomes imaginary close to Γ along K-Γ direction, which indicates instability of the flat 2D graphene sheets.
Exploiting molecular dynamics in Nested Sampling simulations of small peptides
NASA Astrophysics Data System (ADS)
Burkoff, Nikolas S.; Baldock, Robert J. N.; Várnai, Csilla; Wild, David L.; Csányi, Gábor
2016-04-01
Nested Sampling (NS) is a parameter space sampling algorithm which can be used for sampling the equilibrium thermodynamics of atomistic systems. NS has previously been used to explore the potential energy surface of a coarse-grained protein model and has significantly outperformed parallel tempering when calculating heat capacity curves of Lennard-Jones clusters. The original NS algorithm uses Monte Carlo (MC) moves; however, a variant, Galilean NS, has recently been introduced which allows NS to be incorporated into a molecular dynamics framework, so NS can be used for systems which lack efficient prescribed MC moves. In this work we demonstrate the applicability of Galilean NS to atomistic systems. We present an implementation of Galilean NS using the Amber molecular dynamics package and demonstrate its viability by sampling alanine dipeptide, both in vacuo and implicit solvent. Unlike previous studies of this system, we present the heat capacity curves of alanine dipeptide, whose calculation provides a stringent test for sampling algorithms. We also compare our results with those calculated using replica exchange molecular dynamics (REMD) and find good agreement. We show the computational effort required for accurate heat capacity estimation for small peptides. We also calculate the alanine dipeptide Ramachandran free energy surface for a range of temperatures and use it to compare the results using the latest Amber force field with previous theoretical and experimental results.
Random Matrix Theory in molecular dynamics analysis.
Palese, Luigi Leonardo
2015-01-01
It is well known that, in some situations, principal component analysis (PCA) carried out on molecular dynamics data results in the appearance of cosine-shaped low index projections. Because this is reminiscent of the results obtained by performing PCA on a multidimensional Brownian dynamics, it has been suggested that short-time protein dynamics is essentially nothing more than a noisy signal. Here we use Random Matrix Theory to analyze a series of short-time molecular dynamics experiments which are specifically designed to be simulations with high cosine content. We use as a model system the protein apoCox17, a mitochondrial copper chaperone. Spectral analysis on correlation matrices allows to easily differentiate random correlations, simply deriving from the finite length of the process, from non-random signals reflecting the intrinsic system properties. Our results clearly show that protein dynamics is not really Brownian also in presence of the cosine-shaped low index projections on principal axes.
NASA Astrophysics Data System (ADS)
Montoya-Castillo, Andrés; Reichman, David R.
2017-02-01
The ability to efficiently and accurately calculate equilibrium time correlation functions of many-body condensed phase quantum systems is one of the outstanding problems in theoretical chemistry. The Nakajima-Zwanzig-Mori formalism coupled to the self-consistent solution of the memory kernel has recently proven to be highly successful for the computation of nonequilibrium dynamical averages. Here, we extend this formalism to treat symmetrized equilibrium time correlation functions for the spin-boson model. Following the first paper in this series [A. Montoya-Castillo and D. R. Reichman, J. Chem. Phys. 144, 184104 (2016)], we use a Dyson-type expansion of the projected propagator to obtain a self-consistent solution for the memory kernel that requires only the calculation of normally evolved auxiliary kernels. We employ the approximate mean-field Ehrenfest method to demonstrate the feasibility of this approach. Via comparison with numerically exact results for the correlation function Cz z(t ) =Re ⟨σz(0 ) σz(t ) ⟩ , we show that the current scheme affords remarkable boosts in accuracy and efficiency over bare Ehrenfest dynamics. We further explore the sensitivity of the resulting dynamics to the choice of kernel closures and the accuracy of the initial canonical density operator.
Dynamic Wetting in a Non-Equilibrium Gas: The Effect of Gas Pressure on Air Entrainment
NASA Astrophysics Data System (ADS)
Sprittles, James
2014-11-01
Experimentally, it is now well established that lowering the pressure of an ambient gas can suppress wetting failures, or ``air entrainment,'' at a liquid-solid-gas moving contact-line in both coating processes and drop impact dynamics. In this work, we consider the possibility that non-equilibrium effects in the gas are responsible for such phenomena. These can be included into a continuum framework by allowing for slip at both the solid-gas and liquid-gas interfaces, caused by Knudsen layers attached to these boundaries, which is related to the mean free path in the gas, and hence the ambient pressure. This model has been incorporated into a computational framework developed for dynamic wetting phenomena, which resolves all scales in the problem, so that these new effects can be investigated. It is shown that reductions in gas pressure, and hence increases in slip, can dramatically modify the flow field in the gas-film in front of a moving contact-line so that air entrainment is prevented. Specifically, in a dip-coating setup it is shown that the new model (a) describes experimental results for the critical wetting speed at a given gas pressure and (b) allows us to identify new parameters associated with the non-equilibrium gas dynamics which govern the contact-line's motion.
Montoya-Castillo, Andrés; Reichman, David R
2017-02-28
The ability to efficiently and accurately calculate equilibrium time correlation functions of many-body condensed phase quantum systems is one of the outstanding problems in theoretical chemistry. The Nakajima-Zwanzig-Mori formalism coupled to the self-consistent solution of the memory kernel has recently proven to be highly successful for the computation of nonequilibrium dynamical averages. Here, we extend this formalism to treat symmetrized equilibrium time correlation functions for the spin-boson model. Following the first paper in this series [A. Montoya-Castillo and D. R. Reichman, J. Chem. Phys. 144, 184104 (2016)], we use a Dyson-type expansion of the projected propagator to obtain a self-consistent solution for the memory kernel that requires only the calculation of normally evolved auxiliary kernels. We employ the approximate mean-field Ehrenfest method to demonstrate the feasibility of this approach. Via comparison with numerically exact results for the correlation function Czz(t)=Re⟨σz(0)σz(t)⟩, we show that the current scheme affords remarkable boosts in accuracy and efficiency over bare Ehrenfest dynamics. We further explore the sensitivity of the resulting dynamics to the choice of kernel closures and the accuracy of the initial canonical density operator.
Non-equilibrium dynamics of an impurity in the one-dimensional Bose gas
NASA Astrophysics Data System (ADS)
Robinson, Neil; Konik, Robert
2015-03-01
In recent years, the out-of-equilibrium dynamics of interacting many-body quantum systems have attracted much attention. Integrable quantum models have played an important role in understanding the role of local conservation laws in the relaxation of observables, explaining unusual experimental observations in the one-component Bose gas. We study the non-equilibrium dynamics of ``impurity'' wave packets containing a single boson propagating in the one-component Bose gas. Utilizing the integrability of the multi-component Lieb-Liniger model and recent results from the algebraic Bethe ansatz, we compute the time-evolution of the density profile of the ``impurity'' in the cases where the bosons is of the same or different species as the background gas. Our method, based upon numerically solving the Bethe ansatz equations and evaluating the Lehmann spectral representation for local observables, allows us to reach long times with high numerical precision. By comparing results from the two-component Lieb-Liniger model to the one-component Bose gas we can comment on the role of distinguishability in the dynamics of impurities in integrable models.
Non-Equilibrium Dynamics of C-QED Arrays in Strong Correlation Regime
NASA Astrophysics Data System (ADS)
Zhang, Xin-Ding; Li, Zhi-Hang; Zhang, Xiao-Ming
2016-11-01
Recently increasing interests are attracted in the physics of controlled arrays of nonlinear cavity resonators because of the rapid experimental progress achieved in cavity and circuit quantum electrodynamics (QED). For a driven-dissipative two-dimentional planar C-QED array, standard Markov master equation is generally used to study the dynamics of this system. However, when in the case that the on-site photon-photon interaction enters strong correlation regime, standard Markov master equation may lead to incorrect results. In this paper we study the non-equilibrium dynamics of a two-dimentional C-QED array, which is homogeneously pumped by an external pulse, at the same time dissipation exits. We study the evolution of the average photon number of a single cavity by deriving a modified master equation to. In comparison with the standard master equation, the numerical result obtained by our newly derived master equation shows significant difference for the non-equilibrium dynamics of the system.
Heat and mass transfer in reacting mixtures: Molecular dynamics and kinetic theory approaches
NASA Astrophysics Data System (ADS)
Kustova, E.; Nabokova, M.; Kjelstrup, S.; Bedeaux, D.
2016-11-01
Transport properties of a binary H2-H mixture with strongly-non-equilibrium dissociation reaction are studied on the basis of two approaches: kinetic theory and molecular dynamics. The gas in the thermostat under the action of temperature gradient is considered. Mass diffusive and measurable heat flux are obtained in the non-equilibrium molecular dynamics simulations; the transport coefficients are extracted from the fluxes using the constitutive equations given by irreversible thermodynamics. For the same conditions, the transport coefficients and the corresponding fluxes are calculated using the modified Chapman-Enskog method for the rarefied flows with non-equilibrium chemical reactions. While the qualitative agreement between the results obtained using the two approaches is found, quantitative differences are however noticeable. The discrepancy in the heat conductivity coefficient is not large but is significant for diffusion coefficients. Possible sources of discrepancies are discussed.
Molecular dynamics simulation of an argon cluster filled inside carbon nanotubes
NASA Astrophysics Data System (ADS)
Cui, Shu-Wen; Zhu, Ru-Zeng; Wang, Xiao-Song; Yang, Hong-Xiu
2014-10-01
The effects of the diameters of single-walled carbon nanotubes (SWCNTs) (7.83 Å to 27.40 Å) and temperature (20 K-45 K) on the equilibrium structure of an argon cluster are systematically studied by molecular dynamics simulation with consideration of the SWCNTs to be fixed. Since the diameters of SWCNTs with different chiralities increase when temperature is fixed at 20 K, the equilibrium structures of the argon cluster transform from monoatomic chains to helical and then to multishell coaxial cylinders. Chirality has almost no noticeable influence on these cylindrosymmetric structures. The effects of temperature and a non-equilibrium sudden heating process on the structures of argon clusters in SWCNTs are also studied by molecular dynamics simulation.
Chara, Osvaldo; Grigera, José Raúl; McCarthy, Andrés N
2007-12-01
The usefulness of computational methods such as molecular dynamics simulation has been extensively established for studying systems in equilibrium. Nevertheless, its application to complex non-equilibrium biological processes such as protein unfolding has been generally regarded as producing results which cannot be interpreted straightforwardly. In the present study, we present results for the kinetics of unfolding of apomyoglobin, based on the analysis of long simulation runs of this protein in solution at 3 kbar (1 atm = 1.01325, bar = 101,325 Pa). We hereby demonstrate that the analysis of the data collected within a simulated time span of 0.18 mus suffices for producing results, which coincide remarkably with the available unfolding kinetics experimental data. This not only validates molecular dynamics simulation as a valuable alternative for studying non-equilibrium processes, but also enables a detailed analysis of the actual structural mechanism which underlies the unfolding process of proteins under elusive denaturing conditions such as high pressure.
Dynamics of excited molecular states
NASA Astrophysics Data System (ADS)
Meyer, Hans-Dieter
2005-01-01
The photo-excitation or photo-ionization of a polyatomic molecule is typically accompanied by a strong excitation of the vibrational modes. In particular when a conical intersection of the electronic potential energy surfaces involved lies within or close to the Frank-Condon zone, the nuclear motion becomes very complicated, often chaotic, and the spectra become irregular and dense. An accurate simulation of the dynamics of such excited molecules requires firstly that the multi-dimensional and multi-state potential energy surface - or a reliable model thereof - can be determined. Secondly, the multi-dimensional quantum dynamics have to be solved. This is a very difficult task, because of the high dimensionality of the problem (6 to 30 degrees of freedom, say). The multi-configuration time-dependent Hartree (MCTDH) method has proven to be very useful for the study of such problems. In fact, an accurate treatment of the quantal dynamics of molecules like the allene cation (C3 H+4, 15D), the butatriene cation (C4 H+4, 18D), or the pyrazine molecule (C4N2H4, 24D) in their full dimensionality, is - up to date - only possible with MCTDH. (The acronym n D denotes the dimensionality.) The construction of the vibronic model Hamiltonian and the MCTDH method will be briefly discussed. After this, the excited state dynamics of the butatriene and pyrazine molecules will be discussed.
Econometrics and data of the 9 sector Dynamic General Equilibrium Model. Volume III. Final report
Berndt, E.R.; Fraumeni, B.M.; Hudson, E.A.; Jorgenson, D.W.; Stoker, T.M.
1981-03-01
This report presents the econometrics and data of the 9 sector Dynamic General Equilibrium Model. There are two key components of 9DGEM - the model of household behavior and the model of produconcrneer behavior. The household model is concerned with decisions on consumption, saving, labor supply and the composition of consumption. The producer model is concerned with output price formation and determination of input patterns and purchases for each of the nine producing sectors. These components form the behavioral basis of DGEM. The remaining components are concerned with constraints, balance conditions, accounting, and government revenues and expenditures (these elements are developed in the report on the model specification).
New directions in fluid dynamics: non-equilibrium aerodynamic and microsystem flows.
Reese, Jason M; Gallis, Michael A; Lockerby, Duncan A
2003-12-15
Fluid flows that do not have local equilibrium are characteristic of some of the new frontiers in engineering and technology, for example, high-speed high-altitude aerodynamics and the development of micrometre-sized fluid pumps, turbines and other devices. However, this area of fluid dynamics is poorly understood from both the experimental and simulation perspectives, which hampers the progress of these technologies. This paper reviews some of the recent developments in experimental techniques and modelling methods for non-equilibrium gas flows, examining their advantages and drawbacks. We also present new results from our computational investigations into both hypersonic and microsystem flows using two distinct numerical methodologies: the direct simulation Monte Carlo method and extended hydrodynamics. While the direct simulation approach produces excellent results and is used widely, extended hydrodynamics is not as well developed but is a promising candidate for future more complex simulations. Finally, we discuss some of the other situations where these simulation methods could be usefully applied, and look to the future of numerical tools for non-equilibrium flows.
Molecular dynamics simulations of substitutional diffusion
Zhou, Xiaowang; Jones, Reese E.; Gruber, Jacob
2016-12-18
In atomistic simulations, diffusion energy barriers are usually calculated for each atomic jump path using a nudged elastic band method. Practical materials often involve thousands of distinct atomic jump paths that are not known a priori. Hence, it is often preferred to determine an overall diffusion energy barrier and an overall pre-exponential factor from the Arrhenius equation constructed through molecular dynamics simulations of mean square displacement of the diffusion species at different temperatures. This approach has been well established for interstitial diffusion, but not for substitutional diffusion at the same confidence. Using In 0.1 Ga 0.9 N as an example,more » we have identified conditions where molecular dynamics simulations can be used to calculate highly converged Arrhenius plots for substitutional alloys. As a result, this may enable many complex diffusion problems to be easily and reliably studied in the future using molecular dynamics, provided that moderate computing resources are available.« less
Molecular dynamics simulations of substitutional diffusion
Zhou, Xiaowang; Jones, Reese E.; Gruber, Jacob
2016-12-18
In atomistic simulations, diffusion energy barriers are usually calculated for each atomic jump path using a nudged elastic band method. Practical materials often involve thousands of distinct atomic jump paths that are not known a priori. Hence, it is often preferred to determine an overall diffusion energy barrier and an overall pre-exponential factor from the Arrhenius equation constructed through molecular dynamics simulations of mean square displacement of the diffusion species at different temperatures. This approach has been well established for interstitial diffusion, but not for substitutional diffusion at the same confidence. Using In 0.1 Ga 0.9 N as an example, we have identified conditions where molecular dynamics simulations can be used to calculate highly converged Arrhenius plots for substitutional alloys. As a result, this may enable many complex diffusion problems to be easily and reliably studied in the future using molecular dynamics, provided that moderate computing resources are available.
Discrete Molecular Dynamics Simulation of Biomolecules
NASA Astrophysics Data System (ADS)
Ding, Feng
2011-10-01
Discrete molecular dynamics (DMD) simulation of hard spheres was the first implementation of molecular dynamics (MD) in history. DMD simulations are computationally more efficient than continuous MD simulations due to simplified interaction potentials. However, also due to these simplified potentials, DMD has often been associated with coarse-grained modeling, and hence continuous MD has become the dominant approach used to study the internal dynamics of biomolecules. With the recent advances in DMD methodology, including the development of high-resolution models for biomolecules and approaches to increase DMD efficiency, DMD simulations are emerging as an important tool in the field of molecular modeling, including the study of protein folding, protein misfolding and aggregation, and protein engineering. Recently, DMD methodology has been applied to modeling RNA folding and protein-ligand recognition. With these improvements to DMD methodology and the continuous increase in available computational power, we expect a growing role of DMD simulations in our understanding of biology.
Liouville-von Neumann molecular dynamics.
Jakowski, Jacek; Morokuma, Keiji
2009-06-14
We present a novel first principles molecular dynamics scheme, called Liouville-von Neumann molecular dynamics, based on Liouville-von Neumann equation for density matrices propagation and Magnus expansion of the time-evolution operator. The scheme combines formally accurate quantum propagation of electrons represented via density matrices and a classical propagation of nuclei. The method requires a few iterations per each time step where the Fock operator is formed and von Neumann equation is integrated. The algorithm (a) is free of constraint and fictitious parameters, (b) avoids diagonalization of the Fock operator, and (c) can be used in the case of fractional occupation as in metallic systems. The algorithm is very stable, and has a very good conservation of energy even in cases when a good quality conventional Born-Oppenheimer molecular dynamics trajectories is difficult to obtain. Test simulations include initial phase of fullerene formation from gaseous C(2) and retinal system.
Liouville-von Neumann molecular dynamics
NASA Astrophysics Data System (ADS)
Jakowski, Jacek; Morokuma, Keiji
2009-06-01
We present a novel first principles molecular dynamics scheme, called Liouville-von Neumann molecular dynamics, based on Liouville-von Neumann equation for density matrices propagation and Magnus expansion of the time-evolution operator. The scheme combines formally accurate quantum propagation of electrons represented via density matrices and a classical propagation of nuclei. The method requires a few iterations per each time step where the Fock operator is formed and von Neumann equation is integrated. The algorithm (a) is free of constraint and fictitious parameters, (b) avoids diagonalization of the Fock operator, and (c) can be used in the case of fractional occupation as in metallic systems. The algorithm is very stable, and has a very good conservation of energy even in cases when a good quality conventional Born-Oppenheimer molecular dynamics trajectories is difficult to obtain. Test simulations include initial phase of fullerene formation from gaseous C2 and retinal system.
NASA Astrophysics Data System (ADS)
Fable, E.; Angioni, C.; Ivanov, A. A.; Lackner, K.; Maj, O.; Medvedev, S. Yu; Pautasso, G.; Pereverzev, G. V.; Treutterer, W.; the ASDEX Upgrade Team
2013-07-01
The modelling of tokamak scenarios requires the simultaneous solution of both the time evolution of the plasma kinetic profiles and of the magnetic equilibrium. Their dynamical coupling involves additional complications, which are not present when the two physical problems are solved separately. Difficulties arise in maintaining consistency in the time evolution among quantities which appear in both the transport and the Grad-Shafranov equations, specifically the poloidal and toroidal magnetic fluxes as a function of each other and of the geometry. The required consistency can be obtained by means of iteration cycles, which are performed outside the equilibrium code and which can have different convergence properties depending on the chosen numerical scheme. When these external iterations are performed, the stability of the coupled system becomes a concern. In contrast, if these iterations are not performed, the coupled system is numerically stable, but can become physically inconsistent. By employing a novel scheme (Fable E et al 2012 Nucl. Fusion submitted), which ensures stability and physical consistency among the same quantities that appear in both the transport and magnetic equilibrium equations, a newly developed version of the ASTRA transport code (Pereverzev G V et al 1991 IPP Report 5/42), which is coupled to the SPIDER equilibrium code (Ivanov A A et al 2005 32nd EPS Conf. on Plasma Physics (Tarragona, 27 June-1 July) vol 29C (ECA) P-5.063), in both prescribed- and free-boundary modes is presented here for the first time. The ASTRA-SPIDER coupled system is then applied to the specific study of the modelling of controlled current ramp-up in ASDEX Upgrade discharges.
Wavelet Analysis for Molecular Dynamics
2015-06-01
factor of 1,000. Ultra-high-molecular-weight polyethylene offers a classic example of the scale challenge: despite its simple chemical makeup , CnH2n+2...below, but also disconnected graphs from individual molecules . 6 Linear homopolymers can be ordered to have block tridiagonal structure where each...solutions for this simple system; r̃(0)1 = √ 2rOH , r̃ (0) 2 = 0, which leads to a symmetric linear molecule , and r̃(0)1 = 0, r̃ (0) 2 = rOH √ 4+2mO
Dynamic signature of molecular association in methanol.
Bertrand, C E; Self, J L; Copley, J R D; Faraone, A
2016-07-07
Quasielastic neutron scattering measurements and molecular dynamics simulations were combined to investigate the collective dynamics of deuterated methanol, CD3OD. In the experimentally determined dynamic structure factor, a slow, non-Fickian mode was observed in addition to the standard density-fluctuation heat mode. The simulation results indicate that the slow dynamical process originates from the hydrogen bonding of methanol molecules. The qualitative behavior of this mode is similar to the previously observed α-relaxation in supercooled water [M. C. Bellissent-Funel et al., Phys. Rev. Lett. 85, 3644 (2000)] which also originates from the formation and dissolution of hydrogen-bonded associates (supramolecular clusters). In methanol, however, this mode is distinguishable well above the freezing transition. This finding indicates that an emergent slow mode is not unique to supercooled water, but may instead be a general feature of hydrogen-bonding liquids and associating molecular liquids.
Molecular scale dynamics of large ring polymers.
Gooßen, S; Brás, A R; Krutyeva, M; Sharp, M; Falus, P; Feoktystov, A; Gasser, U; Pyckhout-Hintzen, W; Wischnewski, A; Richter, D
2014-10-17
We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement. The loop size is an intrinsic property of the ring architecture and is independent of molecular weight. This is the first experimental observation of the space-time evolution of segmental motion in ring polymers illustrating the dynamic consequences of their topology that is unique among all polymeric systems of any other known architecture.
Molecular Scale Dynamics of Large Ring Polymers
NASA Astrophysics Data System (ADS)
Gooßen, S.; Brás, A. R.; Krutyeva, M.; Sharp, M.; Falus, P.; Feoktystov, A.; Gasser, U.; Pyckhout-Hintzen, W.; Wischnewski, A.; Richter, D.
2014-10-01
We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement. The loop size is an intrinsic property of the ring architecture and is independent of molecular weight. This is the first experimental observation of the space-time evolution of segmental motion in ring polymers illustrating the dynamic consequences of their topology that is unique among all polymeric systems of any other known architecture.
NON-EQUILIBRIUM DYNAMICS OF MANY-BODY QUANTUM SYSTEMS: FUNDAMENTALS AND NEW FRONTIER
DeMille, David; LeHur, Karyn
2013-11-27
Rapid progress in nanotechnology and naofabrication techniques has ushered in a new era of quantum transport experiments. This has in turn heightened the interest in theoretical understanding of nonequilibrium dynamics of strongly correlated quantum systems. This project has advanced the frontiers of understanding in this area along several fronts. For example, we showed that under certain conditions, quantum impurities out of equilibrium can be reformulated in terms of an effective equilibrium theory; this makes it possible to use the gamut of tools available for quantum systems in equilibrium. On a different front, we demonstrated that the elastic power of a transmitted microwave photon in circuit QED systems can exhibit a many-body Kondo resonance. We also showed that under many circumstances, bipartite fluctuations of particle number provide an effective tool for studying many-body physics—particularly the entanglement properties of a many-body system. This implies that it should be possible to measure many-body entanglement in relatively simple and tractable quantum systems. In addition, we studied charge relaxation in quantum RC circuits with a large number of conducting channels, and elucidated its relation to Kondo models in various regimes. We also extended our earlier work on the dynamics of driven and dissipative quantum spin-boson impurity systems, deriving a new formalism that makes it possible to compute the full spin density matrix and spin-spin correlation functions beyond the weak coupling limit. Finally, we provided a comprehensive analysis of the nonequilibrium transport near a quantum phase transition in the case of a spinless dissipative resonant-level model. This project supported the research of two Ph.D. students and two postdoctoral researchers, whose training will allow them to further advance the field in coming years.
NASA Astrophysics Data System (ADS)
Bhattacharya, Utso; Dutta, Amit
2017-05-01
We study the behavior of Fisher zeros (FZs) and dynamical quantum phase transitions (DQPTs) for a linearly ramped Haldane model occurring in the subsequent temporal evolution of the same, and we probe the intimate connection with the equilibrium topology of the model. Here, we investigate the temporal evolution of the final state of the Haldane Hamiltonian (evolving with the time-independent final Hamiltonian) reached following a linear ramping of the staggered (Semenoff) mass term from an initial to a final value, first selecting a specific protocol, so chosen that the system is ramped from one nontopological phase to the other through a topological phase. We establish the existence of three possible behaviors of areas of FZs corresponding to a given sector: (i) no-DQPT, (ii) one-DQPT (intermediate), and (iii) two-DQPTs (reentrant), depending on the inverse quenching rate τ . Our study also reveals that the appearance of the areas of FZs is an artefact of the nonzero (quasi-momentum-dependent) Haldane mass (MH), whose absence leads to an emergent one-dimensional behavior indicated by the shrinking of the area's FZs to lines and the nonanalyticity in the dynamical "free energy" itself. Moreover, the characteristic rates of crossover between the three behaviors of FZs are determined by the time-reversal-invariant quasimomentum points of the Brillouin zone where MH vanishes. Thus, we observe that through the presence or absence of MH, there exists an intimate relation to the topological properties of the equilibrium model even when the ramp drives the system far away from equilibrium.
Testing a Dynamical Equilibrium Model of the Extraplanar Diffuse Ionized Gas in NGC 891
NASA Astrophysics Data System (ADS)
Boettcher, Erin; Zweibel, Ellen G.; Gallagher, J. S., III; Benjamin, Robert A.
2016-12-01
The observed scale heights of extraplanar diffuse ionized gas (eDIG) layers exceed their thermal scale heights by a factor of a few in the Milky Way and other nearby edge-on disk galaxies. Here, we test a dynamical equilibrium model of the eDIG layer in NGC 891, where we ask whether the thermal, turbulent, magnetic field, and cosmic-ray pressure gradients are sufficient to support the layer. In optical emission-line spectroscopy from the SparsePak integral field unit on the WIYN 3.5 m telescope, the Hα emission in position-velocity space suggests that the eDIG is found in a ring between galactocentric radii of {R}\\min ≤slant R≤slant 8 {kpc}, where {R}\\min ≥slant 2 {kpc}. We find that the thermal ({σ }{th}=11 km s-1) and turbulent ({σ }{turb}=25 km s-1) velocity dispersions are insufficient to satisfy the hydrostatic equilibrium equation given an exponential electron scale height of {h}z=1.0 {kpc}. Using a literature analysis of radio continuum observations from the CHANG-ES survey, we demonstrate that the magnetic field and cosmic-ray pressure gradients are sufficient to stably support the gas at R≥slant 8 kpc if the cosmic rays are sufficiently coupled to the system ({γ }{cr}=1.45). Thus, a stable dynamical equilibrium model is viable only if the eDIG is found in a thin ring around R = 8 kpc, and nonequilibrium models such as a galactic fountain flow are of interest for further study.
NASA Astrophysics Data System (ADS)
Blersch, S. S.; Habberfield, M.
2009-12-01
Dynamic equilibrium is a concept that ecologists and fluvial geomorphologists readily use to describe natural systems in their respective fields. The existence of numerous rapid assessment techniques to quantify the dynamic nature of streams, however, shows that consensus has not yet been reached on what a guiding image might be. A brief review of these techniques shows a disparity of perspective between the various academic disciplines that focus on streams; indeed, traditional biology has its index of biotic integrity, geomorphology has its channel stability schemes, and hydrology has its discharge relationship curves. Reviewing these assessment techniques in the context of general systems theory provides a means to select techniques that will inform the dynamic image of the river in its ecologically restored state. In terms of assessment for restoration planning, on which elements of the signature should one focus—the drivers (e.g. the physical properties of the stream) or the responses (e.g. the biological community)? What is the appropriate scale at which one should be working within a stream to determine its state of dynamic equilibrium? To answer these questions, visual-based assessments techniques were compared and contrasted at a potential stream restoration site at an active gravel mining area in Elton Creek in Western New York. Two techniques focused on channel stability only, while the third included channel stability and biological indicators. A comparison was made to determine if different rankings would result using each method, and whether one or the other was more advantageous to establishing restoration design criteria. The two channel stability methods ranked all four reaches tested in the same order, while the biological assessment ranked two of the four differently. The two reaches that did not have the same ranking were fairly close in their overall scores, and were classified in the same category in each method. Those methods with a larger
Molecular dynamic simulations of ocular tablet dissolution.
Ru, Qian; Fadda, Hala M; Li, Chung; Paul, Daniel; Khaw, Peng T; Brocchini, Steve; Zloh, Mire
2013-11-25
Small tablets for implantation into the subconjunctival space in the eye are being developed to inhibit scarring after glaucoma filtration surgery (GFS). There is a need to evaluate drug dissolution at the molecular level to determine how the chemical structure of the active may correlate with dissolution in the nonsink conditions of the conjunctival space. We conducted molecular dynamics simulations to study the dissolution process of tablets derived from two drugs that can inhibit fibrosis after GFS, 5-fluorouracil (5-FU) and the matrix metalloprotease inhibitor (MMPi), ilomastat. The dissolution was simulated in the presence of simple point charge (SPC) water molecules, and the liquid turnover of the aqueous humor in the subconjunctival space was simulated by removal of the dissolved drug molecules at regular intervals and replacement by new water molecules. At the end of the simulation, the total molecular solvent accessible surface area of 5-FU tablets increased by 60 times more than that of ilomastat as a result of tablet swelling and release of molecules into solution. The tablet dissolution pattern shown in our molecular dynamic simulations tends to correlate with experimental release profiles. This work indicates that a series of molecular dynamic simulations can be used to predict the influence of the molecular properties of a drug on its dissolution profile and could be useful during preformulation where sufficient amounts of the drug are not always available to perform dissolution studies.
Understanding Modularity in Molecular Networks Requires Dynamics
Alexander, Roger P.; Kim, Philip M.; Emonet, Thierry; Gerstein, Mark B.
2014-01-01
The era of genome sequencing has produced long lists of the molecular parts from which cellular machines are constructed. A fundamental goal in systems biology is to understand how cellular behavior emerges from the interaction in time and space of genetically encoded molecular parts, as well as non-genetically encoded small molecules. Networks provide a natural framework for the organization and quantitative representation of all the available data about molecular interactions. The structural and dynamic properties of molecular networks have been the subject of intense research. Despite major advances, bridging network structure to dynamics – and therefore to behavior – remains challenging. A key concept of modern engineering that recurs in the functional analysis of biological networks is modularity. Most approaches to molecular network analysis rely to some extent on the assumption that molecular networks are modular – that is, they are separable and can be studied to some degree in isolation. We describe recent advances in the analysis of modularity in biological networks, focusing on the increasing realization that a dynamic perspective is essential to grouping molecules into modules and determining their collective function. PMID:19638611
Modeling the dynamic equilibrium of objects weakened by thin low-strength inclusions
Skopetskii, V.V.; Deineka, V.S.; Marchenko, O.A.
1995-11-01
Successful development of hydroelectric power as well as the use and protection of the resources of the Azov/Black Sea basin require formulation and solution of design and control problems for hydroengineering and coastal constructions. The authors have developed two-dimensional mathematical models of dynamic equilibrium of various hydroengineering and coastal constructions with weak thin sections of natural or artificial origin (low-strength inclusions, cracks, technological seams), where shearing strength conditions must be considered. These models are applicable to objects whose dynamic characteristics can be fully described by considering their profile cross-sections (dams, coastal slopes, wave breakers). The weak thin sections are modeled by cuts with appropriate contact conditions. Finite-element algorithms have been developed for solving the corresponding initial-boundary-value problems, and a model example has been solved.
Pahlavanzadeh, H; Keshtkar, A R; Safdari, J; Abadi, Z
2010-03-15
The biosorption characteristics of nickel(II) ions using the brown algae (Cystoseria indica, Nizmuddinia zanardini, Sargassum glaucescens and Padina australis) were investigated. Experimental parameters affecting the biosorption process such as pH level, contact time, initial metal concentration and temperature were studied. The equilibrium data fitted very well to the Langmuir adsorption model in the concentration range of nickel(II) ions and at all the temperatures studied. Evaluation of the experimental data in terms of biosorption dynamics showed that the biosorption of nickel(II) onto algal biomass followed the pseudo-second-order dynamics well. The calculated thermodynamic parameters (Delta G degrees, Delta H degrees and DeltaS degrees) showed that the biosorption of nickel(II) ions were feasible, spontaneous and endothermic at the temperature ranges of 293-313 K.
Dynamics of Equilibrium Points in a Uniformly Rotating Second-Order and Degree Gravitational Field
NASA Astrophysics Data System (ADS)
Feng, Jinglang; Hou, Xiyun
2017-07-01
Using tools such as periodic orbits and invariant manifolds, the global dynamics around equilibrium points (EPs) in a rotating second-order and degree gravitational field are studied. For EPs on the long axis, planar and vertical periodic families are computed, and their stability properties are investigated. Invariant manifolds are also computed, and their relation to the first-order resonances is briefly discussed. For EPs on the short axis, planar and vertical periodic families are studied, with special emphasis on the genealogy of the planar periodic families. Our studies show that the global dynamics around EPs are highly similar to those around libration points in the circular restricted three-body problem, such as spatial halo orbits, invariant manifolds, and the genealogy of planar periodic families.
Periodic Airy process and equilibrium dynamics of edge fermions in a trap
NASA Astrophysics Data System (ADS)
Doussal, Pierre Le; Majumdar, Satya N.; Schehr, Grégory
2017-08-01
We establish an exact mapping between (i) the equilibrium (imaginary time) dynamics of non-interacting fermions trapped in a harmonic potential at temperature T = 1 / β and (ii) non-intersecting Ornstein-Uhlenbeck (OU) particles constrained to return to their initial positions after time β. Exploiting the determinantal structure of the process we compute the universal correlation functions both in the bulk and at the edge of the trapped Fermi gas. The latter corresponds to the top path of the non-intersecting OU particles, and leads us to introduce and study the time-periodic Airy2 process, A2b (u) , depending on a single parameter, the period b. The standard Airy2 process is recovered for b = + ∞. We discuss applications of our results to the real time quantum dynamics of trapped fermions.
Observation of dynamic equilibrium cluster phase in nanoparticle-polymer system
Kumar, Sugam Mehan, S.; Aswal, V. K.; Schwein, R.
2016-05-23
Small-angle neutron scattering (SANS) and dynamic light scattering (DLS) have been used to investigate the existence of a cluster phase in a nanoparticle-polymer system. The nanoparticle-polymer system shows an interesting reentrant phase behavior where the charge stabilized silica nanoparticles undergo particle clustering and back to individual nanoparticles as a function of polymer concentration. This kind of phase behavior is believed to be directed by opposing attractive and repulsive interactions present in the system. The phase behavior shows two narrow regions of polymer concentration immediately before and after the two-phase formation indicating the possibility of the existence of some equilibrium clusters. DLS results show a much higher size of particles than individuals in these two regions which remains unchanged even after dilution. The SANS data show the evolution of attraction with increased volume fraction of the particles supporting the dynamic nature of these clusters.
Steered Molecular Dynamics Methods Applied to Enzyme Mechanism and Energetics.
Ramírez, C L; Martí, M A; Roitberg, A E
2016-01-01
One of the main goals of chemistry is to understand the underlying principles of chemical reactions, in terms of both its reaction mechanism and the thermodynamics that govern it. Using hybrid quantum mechanics/molecular mechanics (QM/MM)-based methods in combination with a biased sampling scheme, it is possible to simulate chemical reactions occurring inside complex environments such as an enzyme, or aqueous solution, and determining the corresponding free energy profile, which provides direct comparison with experimental determined kinetic and equilibrium parameters. Among the most promising biasing schemes is the multiple steered molecular dynamics method, which in combination with Jarzynski's Relationship (JR) allows obtaining the equilibrium free energy profile, from a finite set of nonequilibrium reactive trajectories by exponentially averaging the individual work profiles. However, obtaining statistically converged and accurate profiles is far from easy and may result in increased computational cost if the selected steering speed and number of trajectories are inappropriately chosen. In this small review, using the extensively studied chorismate to prephenate conversion reaction, we first present a systematic study of how key parameters such as pulling speed, number of trajectories, and reaction progress are related to the resulting work distributions and in turn the accuracy of the free energy obtained with JR. Second, and in the context of QM/MM strategies, we introduce the Hybrid Differential Relaxation Algorithm, and show how it allows obtaining more accurate free energy profiles using faster pulling speeds and smaller number of trajectories and thus smaller computational cost.
Compression, expansion, or dynamic equilibrium? The evolution of health expectancy in New Zealand.
Graham, Patrick; Blakely, Tony; Davis, Peter; Sporle, Andrew; Pearce, Neil
2004-08-01
To evaluate the New Zealand evidence for three theories of population health change: compression of morbidity, expansion of morbidity, and dynamic equilibrium. Using the Sullivan method, repeated cross sectional survey information on functional limitation prevalence was combined with population mortality data and census information on the utilisation of institutional care to produce health expectancy indices for 1981 and 1996. The adult population of New Zealand in 1981 and 1996. 6891 respondents to the 1981 social indicators survey; 8262 respondents to the 1996 household disability survey. As a proportion of overall life expectancy at age 15 the expectation of non-institutionalised mobility limitations increased from 3.5% to 6% for men, and from 4.5% to 8% for women; the expectation of agility limitation increased from 3% to 7.5% for men and from 4.5% to 8.5% for women, and the expectation of self care limitations increased from 2.0% to 4.5% for men and from 3.0% to 6.0% for women. These changes were primarily attributable to increases in the expectation of moderate functional limitation. The dynamic equilibrium scenario provides the best fit to current New Zealand evidence on changes in population health. Although an aging population is likely to lead to an increase in demand for disability support services, the fiscal impact of this increase may be partially offset by a shift from major to moderate limitations, with a consequential reduction in the average levels of support required.
Non-equilibrium solid-to-plasma transition dynamics using XANES diagnostic
NASA Astrophysics Data System (ADS)
Dorchies, F.; Recoules, V.
2016-10-01
The advent of femtosecond lasers has shed new light on non-equilibrium high energy density physics. The ultrafast energy absorption by electrons and the finite rate of their energy transfer to the lattice creates non-equilibrium states of matter, triggering a new class of non-thermal processes from the ambient solid up to extreme conditions of temperature and pressure, referred as the warm dense matter regime. The dynamical interplay between electron and atomic structures is the key issue that drives the ultrafast phase transitions dynamics. Bond weakening or bond hardening are predicted, but strongly depends on the material considered. Many studies have been conducted but this physics is still poorly understood. The experimental tools used up-to-now have provided an incomplete insight. Pure optical techniques measure only indirectly atomic motion through changes in the dielectric function whereas X-ray or electron diffraction only probes the average long-range order. This review is dedicated to recent developments in time-resolved X-ray absorption near-edge spectroscopy, which is expected to give a more complete picture by probing simultaneously the modifications of the near-continuum electron and local atomic structures. Results are reported for three different types of metals (simple, transition and noble metals) in which a confrontation has been carried out between measurements and ab initio simulations.
Klijs, Bart; Mackenbach, Johan P; Kunst, Anton E
2011-04-01
Projections of future trends in the burden of disability could be guided by models linking disability to life expectancy, such as the dynamic equilibrium theory. This article tests the key assumption of this theory that severe disability is associated with proximity to death, whereas mild disability is not. Using data from the GLOBE study (Gezondheid en Levensomstandigheden Bevolking Eindhoven en omstreken), the association of three levels of self-reported disabilities in activities of daily living with age and proximity to death was studied using logistic regression models. Regression estimates were used to estimate the number of life years with disability for life spans of 75 and 85 years. Odds ratios of 0.976 (not significant) for mild disability, 1.137 for moderate disability, and 1.231 for severe disability showed a stronger effect of proximity to death for more severe levels of disability. A 10-year increase of life span was estimated to result in a substantial expansion of mild disability (4.6 years) compared with a small expansion of moderate (0.7 years) and severe (0.9 years) disability. These findings support the theory of a dynamic equilibrium. Projections of the future burden of disability could be substantially improved by connecting to this theory and incorporating information on proximity to death. Copyright © 2011 Elsevier Inc. All rights reserved.
NASA Astrophysics Data System (ADS)
McLaren, Robert; Wojtal, Patryk
2013-04-01
Nitrous acid (HONO) is an important radical precursor in the troposphere. Recent consensus suggests that HONO is formed in the dark through the heterogeneous hydrolysis of NO2 on surfaces (2 NO2 + H2O -> HONO + HNO3), largely dominated by hydrolysis on ground surfaces and a smaller contribution from aerosol surfaces. Frequently a steady state of HONO is observed (dHONO/dt ~ 0) at night, which has been ascribed to a balance between heterogeneous formation and dry deposition. The fate of the surface deposited HONO remains an open question with the possibilities including the permanent loss of N(III), accumulation of surface reservoirs of N(III) at night, and/or a full dynamic equilibrium that partitions HONO between the atmosphere and water on the surface. Surface reservoirs of N(III) accumulated at night could act as a source of HONO the next day. Here we report measurements of HONO by DOAS in Toronto for a 1-year period. Of interest is the frequently observed phenomena of a fast approach to a steady state at sunset with rates of increase of d[HONO]/dt = 1.4 ppb/hr (with snow cover) and long periods in which the mixing ratio of HONO is constant with a median level of ~ 1.1 ppb. We discuss these observations in the context of the steady state being attributable to a dynamic equilibrium state.
Kinetic theory molecular dynamics and hot dense matter: theoretical foundations.
Graziani, F R; Bauer, J D; Murillo, M S
2014-09-01
Electrons are weakly coupled in hot, dense matter that is created in high-energy-density experiments. They are also mildly quantum mechanical and the ions associated with them are classical and may be strongly coupled. In addition, the dynamical evolution of plasmas under these hot, dense matter conditions involve a variety of transport and energy exchange processes. Quantum kinetic theory is an ideal tool for treating the electrons but it is not adequate for treating the ions. Molecular dynamics is perfectly suited to describe the classical, strongly coupled ions but not the electrons. We develop a method that combines a Wigner kinetic treatment of the electrons with classical molecular dynamics for the ions. We refer to this hybrid method as "kinetic theory molecular dynamics," or KTMD. The purpose of this paper is to derive KTMD from first principles and place it on a firm theoretical foundation. The framework that KTMD provides for simulating plasmas in the hot, dense regime is particularly useful since current computational methods are generally limited by their inability to treat the dynamical quantum evolution of the electronic component. Using the N-body von Neumann equation for the electron-proton plasma, three variations of KTMD are obtained. Each variant is determined by the physical state of the plasma (e.g., collisional versus collisionless). The first variant of KTMD yields a closed set of equations consisting of a mean-field quantum kinetic equation for the electron one-particle distribution function coupled to a classical Liouville equation for the protons. The latter equation includes both proton-proton Coulombic interactions and an effective electron-proton interaction that involves the convolution of the electron density with the electron-proton Coulomb potential. The mean-field approach is then extended to incorporate equilibrium electron-proton correlations through the Singwi-Tosi-Land-Sjolander (STLS) ansatz. This is the second variant of KTMD
Numerical methods for molecular dynamics
Skeel, R.D.
1991-01-01
This report summarizes our research progress to date on the use of multigrid methods for three-dimensional elliptic partial differential equations, with particular emphasis on application to the Poisson-Boltzmann equation of molecular biophysics. This research is motivated by the need for fast and accurate numerical solution techniques for three-dimensional problems arising in physics and engineering. In many applications these problems must be solved repeatedly, and the extremely large number of discrete unknowns required to accurately approximate solutions to partial differential equations in three-dimensional regions necessitates the use of efficient solution methods. This situation makes clear the importance of developing methods which are of optimal order (or nearly so), meaning that the number of operations required to solve the discrete problem is on the order of the number of discrete unknowns. Multigrid methods are generally regarded as being in this class of methods, and are in fact provably optimal order for an increasingly large class of problems. The fundamental goal of this research is to develop a fast and accurate numerical technique, based on multi-level principles, for the solutions of the Poisson-Boltzmann equation of molecular biophysics and similar equations occurring in other applications. An outline of the report is as follows. We first present some background material, followed by a survey of the literature on the use of multigrid methods for solving problems similar to the Poisson-Boltzmann equation. A short description of the software we have developed so far is then given, and numerical results are discussed. Finally, our research plans for the coming year are presented.
Semiclassical guided optimal control of molecular dynamics
Kondorskiy, A.; Mil'nikov, G.; Nakamura, H.
2005-10-15
An efficient semiclassical optimal control theory applicable to multidimensional systems is formulated for controlling wave packet dynamics on a single adiabatic potential energy surface. The approach combines advantages of different formulations of optimal control theory: quantum and classical on one hand and global and local on the other. Numerical applications to the control of HCN-CNH isomerization demonstrate that this theory can provide an efficient tool to manipulate molecular dynamics of many degrees of freedom by laser pulses.
Cantu, David C.; Lee, Juntaek; Lee, Mal -Soon; Heldebrant, David J.; Koech, Phillip K.; Freeman, Charles J.; Rousseau, Roger; Glezakou, Vassiliki -Alexandra
2016-03-28
The deployment of transformational non-aqueous CO_{2}-capture solvent systems is encumbered by high viscosity even at intermediate uptakes. Using single-molecule CO_{2} binding organic liquids as a prototypical example, we identify the key molecular features controlling bulk liquid viscosity and CO_{2} uptake kinetics. Fast uptake kinetics arise from close proximity of the alcohol and amine sites that are involved in CO_{2} binding. This process results in the concerted formation of a Zwitterion containing both an alkylcarbonate and a protonated amine. The hydrogen bonding between the two functional groups ultimately determines the solution viscosity. Based on molecular simulation, this work reveals options to significantly reduce viscosity with molecular modifications that shift the proton transfer equilibrium towards a neutral acid/amine species as opposed to the ubiquitously accepted Zwitterionic state. Lastly, the molecular design concepts proposed here, for the alkyl-carbonate systems, are readily extensible to other CO_{2} capture technologies, such as the carbamate- or imidazole-based solvent chemistries.
Cantu, David C.; Lee, Juntaek; Lee, Mal -Soon; ...
2016-03-28
The deployment of transformational non-aqueous CO2-capture solvent systems is encumbered by high viscosity even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we identify the key molecular features controlling bulk liquid viscosity and CO2 uptake kinetics. Fast uptake kinetics arise from close proximity of the alcohol and amine sites that are involved in CO2 binding. This process results in the concerted formation of a Zwitterion containing both an alkylcarbonate and a protonated amine. The hydrogen bonding between the two functional groups ultimately determines the solution viscosity. Based on molecular simulation, this work reveals options tomore » significantly reduce viscosity with molecular modifications that shift the proton transfer equilibrium towards a neutral acid/amine species as opposed to the ubiquitously accepted Zwitterionic state. Lastly, the molecular design concepts proposed here, for the alkyl-carbonate systems, are readily extensible to other CO2 capture technologies, such as the carbamate- or imidazole-based solvent chemistries.« less
Molecular dynamics study of the primary ferrofluid aggregate formation
NASA Astrophysics Data System (ADS)
Tanygin, B. M.; Kovalenko, V. F.; Petrychuk, M. V.; Dzyan, S. A.
2012-11-01
Investigations of the phase transitions and self-organization in the magnetic aggregates are of the fundamental and applied interest. The long-range ordering structures described in the Tománek's systematization (M. Yoon, and D. Tománek, 2010 [1]) are not yet obtained in the direct molecular dynamics simulations. The resulted structures usually are the linear chains or circles, or, else, amorphous (liquid) formations. In the present work, it was shown, that the thermodynamically equilibrium primary ferrofluid aggregate has either the long-range ordered or liquid phase. Due to the unknown steric layer force and other model idealizations, the clear experimental verification of the real equilibrium phase is still required. The predicted long-range ordered (crystallized) phase produces the faceting shape of the primary ferrofluid aggregate, which can be recognized experimentally. The medical (antiviral) application of the crystallized aggregates has been suggested. Dynamic formation of all observed ferrofluid nanostructures conforms to the Tománek's systematization.
Effect of coke in the equilibrium and kinetics of sorption on 5A molecular sieve zeolites
Silva, J.A.C.; Mata, V.G.; Dias, M.M.; Lopes, J.C.B.; Rodrigues, A.E.
2000-04-01
Porosimetric, gravimetric, zero length column (ZLC), and fixed-bed studies on coked pellets of 5A molecular sieve zeolites were performed. From porosimetric studies it seems that the coke is located in the microporous structure of 5A zeolite or any layers covering all crystals. The gravimetric studies between 473 and 573 K using n-pentane as a probe molecule show that Henry's constants in coked pellets are much smaller than those in fresh ones. The kinetics of sorption measured by the ZLC technique is also significantly modified. The results show that the system changes from a macropore control resistance with the reciprocal of time constant D{sub p}/R{sub p}{sup 2}(1 + K) on the order of 0.002--0.02 x{sup {minus}1} in fresh pellets to a micropore control resistance system with reciprocal time constant D{sub c}/r{sub c}{sup 2} 1 order of magnitude lower in coked pellets. The effect of temperature on the behavior of a fixed bed is also shown. A simple mathematical model with equilibrium and diffusivity parameters obtained from independent experiments predicts with good accuracy all fixed-bed adsorption and desorption runs.
Wu, Wei; Wang, Jin
2013-09-28
We established a potential and flux field landscape theory to quantify the global stability and dynamics of general spatially dependent non-equilibrium deterministic and stochastic systems. We extended our potential and flux landscape theory for spatially independent non-equilibrium stochastic systems described by Fokker-Planck equations to spatially dependent stochastic systems governed by general functional Fokker-Planck equations as well as functional Kramers-Moyal equations derived from master equations. Our general theory is applied to reaction-diffusion systems. For equilibrium spatially dependent systems with detailed balance, the potential field landscape alone, defined in terms of the steady state probability distribution functional, determines the global stability and dynamics of the system. The global stability of the system is closely related to the topography of the potential field landscape in terms of the basins of attraction and barrier heights in the field configuration state space. The effective driving force of the system is generated by the functional gradient of the potential field alone. For non-equilibrium spatially dependent systems, the curl probability flux field is indispensable in breaking detailed balance and creating non-equilibrium condition for the system. A complete characterization of the non-equilibrium dynamics of the spatially dependent system requires both the potential field and the curl probability flux field. While the non-equilibrium potential field landscape attracts the system down along the functional gradient similar to an electron moving in an electric field, the non-equilibrium flux field drives the system in a curly way similar to an electron moving in a magnetic field. In the small fluctuation limit, the intrinsic potential field as the small fluctuation limit of the potential field for spatially dependent non-equilibrium systems, which is closely related to the steady state probability distribution functional, is
Cawkwell, M. J. Niklasson, Anders M. N.; Dattelbaum, Dana M.
2015-02-14
The initial chemical events that occur during the shock compression of liquid phenylacetylene have been investigated using self-consistent tight binding molecular dynamics simulations. The extended Lagrangian Born-Oppenheimer molecular dynamics formalism enabled us to compute microcanonical trajectories with precise conservation of the total energy. Our simulations revealed that the first density-increasing step under shock compression arises from the polymerization of phenylacetylene molecules at the acetylene moiety. The application of electronic structure-based molecular dynamics with long-term conservation of the total energy enabled us to identify electronic signatures of reactivity via monitoring changes in the HOMO-LUMO gap, and to capture directly adiabatic shock heating, transient non-equilibrium states, and changes in temperature arising from exothermic chemistry in classical molecular dynamics trajectories.
Equilibrium Statistical Mechanics as a Novel Closure for Atmosphere-Ocean Dynamics
NASA Astrophysics Data System (ADS)
Turkington, B. E.; Majda, A. J.
2001-05-01
rim current that governs the lateral spread of the temperature anomaly; the effects of preconditioning from wind-driven gyres and topography are also realistically represented. In these and other related models, the statistical theory captures the nonlinear behavior of the large scales without detailed resolution of the small scales. Moreover, a corresponding quasi-equilibrium theory can be applied to systems with forcing and dissipation, reducing the full dynamics to the evolution of a few quasi-invariants and thereby providing a novel, dynamically-consistent closure.
Equilibrium sampling by re-weighting non-equilibrium simulation trajectories
NASA Astrophysics Data System (ADS)
Yang, Cheng; Wan, Biao; Xu, Shun; Wang, Yanting
2015-12-01
With the traditional equilibrium molecular simulations, it is usually difficult to efficiently visit the whole conformational space in complex systems, which are separated into some metastable conformational regions by high free energy barriers. The applied non-equilibrium process in simulations could enhance the transitions among these conformational regions, and the associated non-equilibrium effects can be removed by employing the Jarzynski equality (JE), then the global equilibrium distribution can be reproduced. However, the original JE requires the initial distribution of the non-equilibrium process is equilibrium, which largely limits the application of the non-equilibrium method in equilibrium sampling. By extending the previous method, the reweighted ensemble dynamics (RED), which re-weights many equilibrium simulation trajectories from arbitrary initial distribution to reproduce the global equilibrium, to non-equilibrium simulations, we present a method, named as re-weighted non-equilibrium ensemble dynamics (RNED), to generalize the JE in the non-equilibrium trajectories started from an arbitrary initial distribution, thus provide an efficient method to reproduce the equilibrium distribution based on multiple independent (short) non-equilibrium trajectories. We have illustrated the validity of the RNED in a one-dimensional toy model and in a Lennard-Jones system to detect the liquid-solid phase coexistence.
Reaction dynamics in polyatomic molecular systems
Miller, W.H.
1993-12-01
The goal of this program is the development of theoretical methods and models for describing the dynamics of chemical reactions, with specific interest for application to polyatomic molecular systems of special interest and relevance. There is interest in developing the most rigorous possible theoretical approaches and also in more approximate treatments that are more readily applicable to complex systems.
Lugo-Frías, Rodrigo; Klapp, Sabine H L
2016-06-22
This paper is concerned with the dynamics of a binary mixture of rod-like, repulsive colloidal particles driven out of equilibrium by means of a steady shear flow (Couette geometry). To this end we first derive, starting from a microscopic density functional in Parsons-Lee approximation, a mesoscopic free energy functional whose main variables are the orientational order parameter tensors. Based on this mesoscopic functional we then explore the stability of isotropic and nematic equilibrium phases in terms of composition and rod lengths. Second, by combining the equilibrium theory with the Doi-Hess approach for the order parameter dynamics under shear, we investigate the orientational dynamics of binary mixtures for a range of shear rates and coupling parameters. We find a variety of dynamical states, including synchronized oscillatory states of the two components, but also symmetry breaking behavior where the components display different in-plane oscillatory states.
Adaptively restrained molecular dynamics in LAMMPS
NASA Astrophysics Data System (ADS)
Kant Singh, Krishna; Redon, Stephane
2017-07-01
Adaptively restrained molecular dynamics (ARMD) is a recently introduced particles simulation method that switches positional degrees of freedom on and off during simulation in order to speed up calculations. In the NVE ensemble, ARMD allows users to trade between precision and speed while, in the NVT ensemble, it makes it possible to compute statistical averages faster. Despite the conceptual simplicity of the approach, however, integrating it in existing molecular dynamics packages is non-trivial, in particular since implemented potentials should a priori be rewritten to take advantage of frozen particles and achieve a speed-up. In this paper, we present novel algorithms for integrating ARMD in LAMMPS, a popular multi-purpose molecular simulation package. In particular, we demonstrate how to enable ARMD in LAMMPS without having to re-implement all available force fields. The proposed algorithms are assessed on four different benchmarks, and show how they allow us to speed up simulations up to one order of magnitude.
NASA Astrophysics Data System (ADS)
Mohanam, L. N.; Ong, S. W.; Tok, E. S.; Kang, H. C.
2015-02-01
Even though the Car-Parrinello molecular dynamics (CPMD) method provides excellent sampling for thermal equilibrium, coupling between ionic motion and the fictitious orbital dynamics leads to an underestimation of ionic vibration frequencies In this letter we examine how this coupling affects calculated rates of energy-barrier crossing. Simulating double-well potentials in 'toy' models based on H3O2- and H5O2+ complexes, we demonstrate that barrier crossing rates can be dramatically affected depending upon how orbitals evolve at the top of the reaction barrier. Thus, it is important that calculated dynamical quantities such as chemical reaction rates be assessed for this coupling effect.
Excited State Quantum-Classical Molecular Dynamics
NASA Astrophysics Data System (ADS)
Krstic, Predrag
2005-05-01
The development of a new theoretical, algorithmic, and computational framework is reported describing the corresponding excited state many-body dynamics by applying multiphysics described by classical equations of motion for nuclei and Hartree-Fock/Multi-Configuration Hartree-Fock and multiresolution techniques for solving the quantum part of the problem (i.e. the motion of the electrons). We primarily have in mind reactive and electron-transition dynamics which involves molecular clusters, containing hundreds of atoms, perturbed by a slow ionic/atomic/molecular projectile, with possible applications in plasma-surface interactions, cluster physics, chemistry and biotechnology. The validation of the developed technique is performed at three-body systems. Application to the transition dynamics in small carbon clusters and hydrocarbons perturbed by slow carbon ions resolves some long-standing issues in the ion-surface interactions in fusion tokamaks.
Conformational dynamics of the molecular chaperone Hsp90
Krukenberg, Kristin A.; Street, Timothy O.; Lavery, Laura A.; Agard, David A.
2016-01-01
The molecular chaperone Hsp90 is an essential eukaryotic protein that makes up 1–2% of all cytosolic proteins. Hsp90 is vital for the maturation and maintenance of a wide variety of substrate proteins largely involved in signaling and regulatory processes. Many of these substrates have also been implicated in cancer and other diseases making Hsp90 an attractive target for therapeutics. Hsp90 is a highly dynamic and flexible molecule that can adapt its conformation to the wide variety of substrate proteins with which it acts. Large conformational rearrangements are also required for the activation of these client proteins. One driving force for these rearrangements is the intrinsic ATPase activity of Hsp90, as seen with other chaperones. However, unlike other chaperones, studies have shown that the ATPase cycle of Hsp90 is not conformationally deterministic. That is, rather than dictating the conformational state, ATP binding and hydrolysis shifts the equilibrium between a pre-existing set of conformational states in an organism-dependent manner. In vivo Hsp90 functions as part of larger heterocomplexes. The binding partners of Hsp90, co-chaperones, assist in the recruitment and activation of substrates, and many co-chaperones further regulate the conformational dynamics of Hsp90 by shifting the conformational equilibrium towards a particular state. Studies have also suggested alternative mechanisms for the regulation of Hsp90’s conformation. In this review, we discuss the structural and biochemical studies leading to our current understanding of the conformational dynamics of Hsp90 and the role that nucleotide, co-chaperones, post-translational modification and clients play in regulating Hsp90’s conformation. We also discuss the effects of current Hsp90 inhibitors on conformation and the potential for developing small molecules that inhibit Hsp90 by disrupting the conformational dynamics. PMID:21414251
Methods for molecular dynamics simulations of protein folding/unfolding in solution.
Beck, David A C; Daggett, Valerie
2004-09-01
All atom molecular dynamics simulations have become a standard method for mapping equilibrium protein dynamics and non-equilibrium events like folding and unfolding. Here, we present detailed methods for performing such simulations. Generic protocols for minimization, solvation, simulation, and analysis derived from previous studies are also presented. As a measure of validation, our water model is compared with experiment. An example of current applications of these methods, simulations of the ultrafast folding protein Engrailed Homeodomain are presented including the experimental evidence used to verify their results. Ultrafast folders are an invaluable tool for studying protein behavior as folding and unfolding events measured by experiment occur on timescales accessible with the high-resolution molecular dynamics methods we describe. Finally, to demonstrate the prospect of these methods for folding proteins, a temperature quench simulation of a thermal unfolding intermediate of the Engrailed Homeodomain is described.
Dynamic covalent chemistry approaches toward macrocycles, molecular cages, and polymers.
Jin, Yinghua; Wang, Qi; Taynton, Philip; Zhang, Wei
2014-05-20
The current research in the field of dynamic covalent chemistry includes the study of dynamic covalent reactions, catalysts, and their applications. Unlike noncovalent interactions utilized in supramolecular chemistry, the formation/breakage of covalent bonding has slower kinetics and usually requires the aid of a catalyst. Catalytic systems that enable efficient thermodynamic equilibrium are thus essential. In this Account, we describe the development of efficient catalysts for alkyne metathesis, and discuss the application of dynamic covalent reactions (mainly imine, olefin, and alkyne metathesis) in the development of organic functional materials. Alkyne metathesis is an emerging dynamic covalent reaction that offers robust and linear acetylene linkages. By introducing a podand motif into the catalyst ligand design, we have developed a series of highly active and robust alkyne metathesis catalysts, which, for the first time, enabled the one-step covalent assembly of ethynylene-linked functional molecular cages. Imine chemistry and olefin metathesis are among the most well-established reversible reactions, and have also been our main synthetic tools. Various shape-persistent macrocycles and covalent organic polyhedrons have been efficiently constructed in one-step through dynamic imine chemistry and olefin metathesis. The geometrical features and solubilizing groups of the building blocks as well as the reaction kinetics have significant effect on the outcome of a covalent assembly process. More recently, we explored the orthogonality of imine and olefin metatheses, and successfully synthesized heterosequenced macrocycles and molecular cages through one-pot orthogonal dynamic covalent chemistry. In addition to discrete molecular architectures, functional polymeric materials can also be accessed through dynamic covalent reactions. Defect-free solution-processable conjugated polyaryleneethynylenes and polydiacetylenes have been prepared through alkyne metathesis
Cantu, David C; Lee, Juntaek; Lee, Mal-Soon; Heldebrant, David J; Koech, Phillip K; Freeman, Charles J; Rousseau, Roger; Glezakou, Vassiliki-Alexandra
2016-05-05
The deployment of transformational nonaqueous CO2-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO2 capture technologies.
NASA Astrophysics Data System (ADS)
Pedesseau, Laurent; Jouanna, Paul
2004-12-01
The SASP (semianalytical stochastic perturbations) method is an original mixed macro-nano-approach dedicated to the mass equilibrium of multispecies phases, periphases, and interphases. This general method, applied here to the reflexive relation Ck⇔μk between the concentrations Ck and the chemical potentials μk of k species within a fluid in equilibrium, leads to the distribution of the particles at the atomic scale. The macroaspects of the method, based on analytical Taylor's developments of chemical potentials, are intimately mixed with the nanoaspects of molecular mechanics computations on stochastically perturbed states. This numerical approach, directly linked to definitions, is universal by comparison with current approaches, DLVO Derjaguin-Landau-Verwey-Overbeek, grand canonical Monte Carlo, etc., without any restriction on the number of species, concentrations, or boundary conditions. The determination of the relation Ck⇔μk implies in fact two problems: a direct problem Ck⇒μk and an inverse problem μk⇒Ck. Validation of the method is demonstrated in case studies A and B which treat, respectively, a direct problem and an inverse problem within a free saturated gypsum solution. The flexibility of the method is illustrated in case study C dealing with an inverse problem within a solution interphase, confined between two (120) gypsum faces, remaining in connection with a reference solution. This last inverse problem leads to the mass equilibrium of ions and water molecules within a 3 Å thick gypsum interface. The major unexpected observation is the repulsion of SO42- ions towards the reference solution and the attraction of Ca2+ ions from the reference solution, the concentration being 50 times higher within the interphase as compared to the free solution. The SASP method is today the unique approach able to tackle the simulation of the number and distribution of ions plus water molecules in such extreme confined conditions. This result is of prime
Pedesseau, Laurent; Jouanna, Paul
2004-12-22
The SASP (semianalytical stochastic perturbations) method is an original mixed macro-nano-approach dedicated to the mass equilibrium of multispecies phases, periphases, and interphases. This general method, applied here to the reflexive relation C(k)<=>mu(k) between the concentrations C(k) and the chemical potentials mu(k) of k species within a fluid in equilibrium, leads to the distribution of the particles at the atomic scale. The macroaspects of the method, based on analytical Taylor's developments of chemical potentials, are intimately mixed with the nanoaspects of molecular mechanics computations on stochastically perturbed states. This numerical approach, directly linked to definitions, is universal by comparison with current approaches, DLVO Derjaguin-Landau-Verwey-Overbeek, grand canonical Monte Carlo, etc., without any restriction on the number of species, concentrations, or boundary conditions. The determination of the relation C(k)<=>mu(k) implies in fact two problems: a direct problem C(k)=>mu(k) and an inverse problem mu(k)=>C(k). Validation of the method is demonstrated in case studies A and B which treat, respectively, a direct problem and an inverse problem within a free saturated gypsum solution. The flexibility of the method is illustrated in case study C dealing with an inverse problem within a solution interphase, confined between two (120) gypsum faces, remaining in connection with a reference solution. This last inverse problem leads to the mass equilibrium of ions and water molecules within a 3 A thick gypsum interface. The major unexpected observation is the repulsion of SO(4) (2-) ions towards the reference solution and the attraction of Ca(2+) ions from the reference solution, the concentration being 50 times higher within the interphase as compared to the free solution. The SASP method is today the unique approach able to tackle the simulation of the number and distribution of ions plus water molecules in such extreme confined conditions
Temperature fluctuations in canonical systems: Insights from molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Hickman, J.; Mishin, Y.
2016-11-01
Molecular dynamics simulations of a quasiharmonic solid are conducted to elucidate the meaning of temperature fluctuations in canonical systems and validate a well-known but frequently contested equation predicting the mean square of such fluctuations. The simulations implement two virtual and one physical (natural) thermostat and examine the kinetic, potential, and total energy correlation functions in the time and frequency domains. The results clearly demonstrate the existence of quasiequilibrium states in which the system can be characterized by a well-defined temperature that follows the mentioned fluctuation equation. The emergence of such states is due to the wide separation of time scales between thermal relaxation by phonon scattering and slow energy exchanges with the thermostat. The quasiequilibrium states exist between these two time scales when the system behaves as virtually isolated and equilibrium.
Thermodynamic properties of gold-water nanofluids using molecular dynamics
NASA Astrophysics Data System (ADS)
Puliti, Gianluca; Paolucci, Samuel; Sen, Mihir
2012-12-01
The physical behavior of a nanofluids is still not fully understood. This work focuses on the study and understanding of equilibrium thermodynamic properties of several gold-water nanofluids using molecular dynamics simulations. Three different systems are considered, where gold nanoparticles with diameters of 2.6, 5.8, and 6.6 nm are suspended in water for effective nanoparticle volume fractions of 1, 10, and 15 %, respectively. Novelties of this study are in the use of accurate force fields for modeling the inter- and intramolecular interactions of the components, and providing comprehensive thermodynamic properties of the nanofluids. The results are validated with the pure fluid and solid properties. Results indicate that the thermodynamics of the system does not behave as an ideal mixture, due to a combination of several factors such as liquid layering, anisotropicity, and other solid-liquid interfacial effects.
Molecular dynamics study of a polymeric reverse osmosis membrane.
Harder, E.; Walters, D. E.; Bodnar, Y. D.; Faibish, R. S.; Roux, B.
2009-07-30
Molecular dynamics (MD) simulations are used to investigate the properties of an atomic model of an aromatic polyamide reverse osmosis membrane. The monomers forming the polymeric membrane are cross-linked progressively on the basis of a heuristic distance criterion during MD simulations until the system interconnectivity reaches completion. Equilibrium MD simulations of the hydrated membrane are then used to determine the density and diffusivity of water within the membrane. Given a 3 MPa pressure differential and a 0.125 {micro}m width membrane, the simulated water flux is calculated to be 1.4 x 10{sup -6} m/s, which is in fair agreement with an experimental flux measurement of 7.7 x 10{sup -6} m/s.
Molecular Dynamics Simulations of Anti-Aggregation Effect of Ibuprofen
Chang, Wenling E.; Takeda, Takako; Raman, E. Prabhu; Klimov, Dmitri K.
2010-01-01
Abstract Using implicit solvent molecular dynamics and replica exchange simulations, we study the impact of ibuprofen on the growth of wild-type Aβ fibrils. We show that binding of ibuprofen to Aβ destabilizes the interactions between incoming peptides and the fibril. As a result, ibuprofen interference modifies the free energy landscape of fibril growth and reduces the free energy gain of Aβ peptide binding to the fibril by ≃2.5 RT at 360 K. Furthermore, ibuprofen interactions shift the thermodynamic equilibrium from fibril-like locked states to disordered docked states. Ibuprofen's anti-aggregation effect is explained by its competition with incoming Aβ peptides for the same binding site located on the fibril edge. Although ibuprofen impedes fibril growth, it does not significantly change the mechanism of fibril elongation or the structure of Aβ peptides bound to the fibril. PMID:20513411
Thermal conductivity of penta-graphene from molecular dynamics study.
Xu, Wen; Zhang, Gang; Li, Baowen
2015-10-21
Using classical equilibrium molecular dynamics simulations and applying the original Tersoff interatomic potential, we study the thermal transport property of the latest two dimensional carbon allotrope, penta-graphene. It is predicted that its room-temperature thermal conductivity is about 167 W/mK, which is much lower than that of graphene. With normal mode decomposition, the accumulated thermal conductivity with respect to phonon frequency and mean free path is analyzed. It is found that the acoustic phonons make a contribution of about 90% to the thermal conductivity, and phonons with mean free paths larger than 100 nm make a contribution over 50%. We demonstrate that the remarkably lower thermal conductivity of penta-graphene compared with graphene results from the lower phonon group velocities and fewer collective phonon excitations. Our study highlights the importance of structure-property relationship and provides better understanding of thermal transport property and valuable insight into thermal management of penta-graphene.
Relationship between nanocrystalline and amorphous microstructures by molecular dynamics simulation
Keblinski, P.; Phillpot, S.R.; Wolf, D.; Gleiter, H.
1996-08-01
A recent molecular dynamics simulation method for growth of fully dense nanocrystalline materials crystallized from melt was used with the Stillinger-Weber three-body potential to synthesize nanocrystalline Si with a grain size up to 75{Angstrom}. Structures of the highly constrained grain boundaries (GBs), triple lines, and point grain junctions were found to be highly disordered and similar to the structure of amorphous Si. These and earlier results for fcc metals suggest that a nanocrystalline microstructure may be viewed as a two-phase system, namely an ordered crystalline phase in the grain interiors connected by an amorphous, intergranular, glue-like phase. Analysis of the structures of bicrystalline GBs in the same materials reveals the presence of an amorphous intergranular equilibrium phase only in the high-energy but not the low-energy GBs, suggesting that only high-energy boundaries are present in nanocrystalline microstructures.
Fragmentation of silicon microclusters: A molecular-dynamics study
NASA Astrophysics Data System (ADS)
Feuston, B. P.; Kalia, R. K.; Vashishta, P.
1987-04-01
A detailed molecular-dynamics (MD) study has been performed to (i) enumerate ground-state and finite-temperature structures and (ii) investigate the nature of fragmentation for SiN clusters (N=2-14), using the Stillinger-Weber three-body interaction potential. We investigate all underlying mechanically stable structures visited by the system in the equilibrium state. Results indicate that the presence of magic numbers, unusually stable finite-temperature clusters Si4, Si6, and Si10 as determined by the photofragmentation experiment of Bloomfield, Freeman, and Brown is dependent upon the topology and energetics of high-energy bound structures rather than upon the structure and ground-state energies at zero temperature.
Molecular dynamics study of sound propagation in a gas
NASA Astrophysics Data System (ADS)
Yano, Takeru
2012-09-01
Large scale molecular dynamics simulations have been performed to clarify the nonlinear and non-equilibrium processes of large-amplitude and high-frequency progressive sound waves in a gas. The statistical errors in calculating macroscopic variables are reduced by putting more than 0.3 million molecules in a simulation box with the length of several micrometers in the direction of wave propagation. The one-dimensional sound wave is generated by a harmonic oscillation of sound source with the Lennard-Jones intermolecular potential, which is the same as that of gas molecules. As a result, we find that the large-amplitude and high-frequency sound propagates with strong attenuation, in some cases, exhibiting a stream-like profile accompanied with mass, momentum, and energy transports. This leads to a completely different picture and a different dispersion relation from a classical theory of high-frequency sound based on the linear standing wave analysis.
Fiber Bragg grating dynamic demodulation based on non-equilibrium interferometry
NASA Astrophysics Data System (ADS)
Yu, Qi; Jing, Zhenguo; Peng, Wei; Zhang, Xinpu; Liu, Yun; Xing, Chuanqi; Li, Hong; Yao, Wenjuan
2011-12-01
Non-equilibrium interferometric Fiber Bragg Grating (FBG) sensor is suitable for the accurate measurements of high-frequency dynamic stress, vibration, etc because of its high sensitivity and high frequency response compared to other types of FBG sensors. In this paper, a Phase Generation Carrier (PGC) demodulation technique of non-equilibrium interferometric FBG sensor that based on ARCTAN algorithm by using an arctangent algorithm with a simple method, has been investigated ,which can avoid the high-frequency noise increases, the error accumulation, the integrator signal jump of the integrator and other inherent weaknesses in the system. ARCTAN has a better response characteristic of the mutant signals, especially for low-frequency large-signal that can be demodulated with a greater range. The experimental result demonstrate that implementing measured resolution can up to 10nɛ/√Hz@500Hz in vibration strain, a signal sampling rate to 100 KHz and a frequency response range up to 1 KHz. This method can improve the performance of the system greatly which has potential significance for practical sensor application.
Non-equilibrium entropy and dynamics in a system with long-range interactions
NASA Astrophysics Data System (ADS)
Rocha Filho, T. M.
2016-05-01
We extend the core-halo approach of Levin et al (2014 Phys. Rep. 535, 1) for the violent relaxation of long-range interacting system with a waterbag initial condition, in the case of a widely studied Hamiltonian mean field model. The Gibbs entropy maximization principle is considered with the constraints of energy conservation and of coarse-grained Casimir invariants of the Vlasov equation. The core-halo distribution function depends only on the one-particle mean-field energy, as is expected from the Jeans theorem, and depends on a set of parameters which in our approach is completely determined without having to solve an envelope equation for the contour of the initial state, as required in the original approach. We also show that a different ansatz can be used for the core-halo distribution with similar results. This work also reveals a link between a parametric resonance causing the non-equilibrium phase transition in the model, a dynamical property, and a discontinuity of the (non-equilibrium) entropy of the system.
Polarity, cell division, and out-of-equilibrium dynamics control the growth of epithelial structures
Cerruti, Benedetta; Puliafito, Alberto; Shewan, Annette M.; Yu, Wei; Combes, Alexander N.; Little, Melissa H.; Chianale, Federica; Primo, Luca; Serini, Guido; Mostov, Keith E.; Celani, Antonio
2013-01-01
The growth of a well-formed epithelial structure is governed by mechanical constraints, cellular apico-basal polarity, and spatially controlled cell division. Here we compared the predictions of a mathematical model of epithelial growth with the morphological analysis of 3D epithelial structures. In both in vitro cyst models and in developing epithelial structures in vivo, epithelial growth could take place close to or far from mechanical equilibrium, and was determined by the hierarchy of time-scales of cell division, cell–cell rearrangements, and lumen dynamics. Equilibrium properties could be inferred by the analysis of cell–cell contact topologies, and the nonequilibrium phenotype was altered by inhibiting ROCK activity. The occurrence of an aberrant multilumen phenotype was linked to fast nonequilibrium growth, even when geometric control of cell division was correctly enforced. We predicted and verified experimentally that slowing down cell division partially rescued a multilumen phenotype induced by altered polarity. These results improve our understanding of the development of epithelial organs and, ultimately, of carcinogenesis. PMID:24145168
Takeuchi, Koh; Imai, Misaki; Shimada, Ichio
2017-03-21
LmrR is a multidrug binding transcriptional repressor that controls the expression of a major multidrug transporter, LmrCD, in Lactococcus lactis. Promiscuous compound ligations reduce the affinity of LmrR for the lmrCD operator by several fold to release the transcriptional repression; however, the affinity reduction is orders of magnitude smaller than that of typical transcriptional repressors. Here, we found that the transcriptional regulation of LmrR is achieved through an equilibrium between the operator-bound and non-specific DNA-adsorption states in vivo. The effective dissociation constant of LmrR for the lmrCD operator under the equilibrium is close to the endogenous concentration of LmrR, which allows a substantial reduction of LmrR occupancy upon compound ligations. Therefore, LmrR represents a dynamic type of transcriptional regulation of prokaryotic multidrug resistance systems, where the small affinity reduction induced by compounds is coupled to the functional relocalization of the repressor on the genomic DNA via nonspecific DNA adsorption.
Mac Low, Mordecai-Mark; Glover, Simon C. O. E-mail: glover@uni-heidelberg.de
2012-02-20
Observations of spiral galaxies show a strong linear correlation between the ratio of molecular to atomic hydrogen surface density R{sub mol} and midplane pressure. To explain this, we simulate three-dimensional, magnetized turbulence, including simplified treatments of non-equilibrium chemistry and the propagation of dissociating radiation, to follow the formation of H{sub 2} from cold atomic gas. The formation timescale for H{sub 2} is sufficiently long that equilibrium is not reached within the 20-30 Myr lifetimes of molecular clouds. The equilibrium balance between radiative dissociation and H{sub 2} formation on dust grains fails to predict the time-dependent molecular fractions we find. A simple, time-dependent model of H{sub 2} formation can reproduce the gross behavior, although turbulent density perturbations increase molecular fractions by a factor of few above it. In contradiction to equilibrium models, radiative dissociation of molecules plays little role in our model for diffuse radiation fields with strengths less than 10 times that of the solar neighborhood, because of the effective self-shielding of H{sub 2}. The observed correlation of R{sub mol} with pressure corresponds to a correlation with local gas density if the effective temperature in the cold neutral medium of galactic disks is roughly constant. We indeed find such a correlation of R{sub mol} with density. If we examine the value of R{sub mol} in our local models after a free-fall time at their average density, as expected for models of molecular cloud formation by large-scale gravitational instability, our models reproduce the observed correlation over more than an order-of-magnitude range in density.
Non-equilibrium transport and spin dynamics in single-molecule magnets
NASA Astrophysics Data System (ADS)
Moldoveanu, V.; Dinu, I. V.; Tanatar, B.
2015-11-01
The time-dependent transport through single-molecule magnets (SMM) coupled to magnetic or non-magnetic electrodes is studied in the framework of the generalized Master equation (GME) method. We calculate the transient currents which develop when the molecule is smoothly coupled to the source and drain electrodes. The signature of the electrically induced magnetic switching on these transient currents is investigated. Our simulations show that the magnetic switching of the molecular spin can be read indirectly from the transient currents if one lead is magnetic and it is much faster if the leads have opposite spin polarizations. We identify effects of the transverse anisotropy on the dynamics of molecular states.
Computationally Efficient Multiconfigurational Reactive Molecular Dynamics.
Yamashita, Takefumi; Peng, Yuxing; Knight, Chris; Voth, Gregory A
2012-12-11
It is a computationally demanding task to explicitly simulate the electronic degrees of freedom in a system to observe the chemical transformations of interest, while at the same time sampling the time and length scales required to converge statistical properties and thus reduce artifacts due to initial conditions, finite-size effects, and limited sampling. One solution that significantly reduces the computational expense consists of molecular models in which effective interactions between particles govern the dynamics of the system. If the interaction potentials in these models are developed to reproduce calculated properties from electronic structure calculations and/or ab initio molecular dynamics simulations, then one can calculate accurate properties at a fraction of the computational cost. Multiconfigurational algorithms model the system as a linear combination of several chemical bonding topologies to simulate chemical reactions, also sometimes referred to as "multistate". These algorithms typically utilize energy and force calculations already found in popular molecular dynamics software packages, thus facilitating their implementation without significant changes to the structure of the code. However, the evaluation of energies and forces for several bonding topologies per simulation step can lead to poor computational efficiency if redundancy is not efficiently removed, particularly with respect to the calculation of long-ranged Coulombic interactions. This paper presents accurate approximations (effective long-range interaction and resulting hybrid methods) and multiple-program parallelization strategies for the efficient calculation of electrostatic interactions in reactive molecular simulations.
Computationally Efficient Multiconfigurational Reactive Molecular Dynamics
Yamashita, Takefumi; Peng, Yuxing; Knight, Chris; Voth, Gregory A.
2012-01-01
It is a computationally demanding task to explicitly simulate the electronic degrees of freedom in a system to observe the chemical transformations of interest, while at the same time sampling the time and length scales required to converge statistical properties and thus reduce artifacts due to initial conditions, finite-size effects, and limited sampling. One solution that significantly reduces the computational expense consists of molecular models in which effective interactions between particles govern the dynamics of the system. If the interaction potentials in these models are developed to reproduce calculated properties from electronic structure calculations and/or ab initio molecular dynamics simulations, then one can calculate accurate properties at a fraction of the computational cost. Multiconfigurational algorithms model the system as a linear combination of several chemical bonding topologies to simulate chemical reactions, also sometimes referred to as “multistate”. These algorithms typically utilize energy and force calculations already found in popular molecular dynamics software packages, thus facilitating their implementation without significant changes to the structure of the code. However, the evaluation of energies and forces for several bonding topologies per simulation step can lead to poor computational efficiency if redundancy is not efficiently removed, particularly with respect to the calculation of long-ranged Coulombic interactions. This paper presents accurate approximations (effective long-range interaction and resulting hybrid methods) and multiple-program parallelization strategies for the efficient calculation of electrostatic interactions in reactive molecular simulations. PMID:25100924
Dynamic Implicit 3D Adaptive Mesh Refinement for Non-Equilibrium Radiation Diffusion
Philip, Bobby; Wang, Zhen; Berrill, Mark A; Rodriguez Rodriguez, Manuel; Pernice, Michael
2014-01-01
The time dependent non-equilibrium radiation diffusion equations are important for solving the transport of energy through radiation in optically thick regimes and find applications in several fields including astrophysics and inertial confinement fusion. The associated initial boundary value problems that are encountered exhibit a wide range of scales in space and time and are extremely challenging to solve. To efficiently and accurately simulate these systems we describe our research on combining techniques that will also find use more broadly for long term time integration of nonlinear multiphysics systems: implicit time integration for efficient long term time integration of stiff multiphysics systems, local control theory based step size control to minimize the required global number of time steps while controlling accuracy, dynamic 3D adaptive mesh refinement (AMR) to minimize memory and computational costs, Jacobian Free Newton Krylov methods on AMR grids for efficient nonlinear solution, and optimal multilevel preconditioner components that provide level independent linear solver convergence.
On the non-equilibrium dynamics of cavitation around the underwater projectile in variable motion
NASA Astrophysics Data System (ADS)
Chen, Y.; Lu, C. J.; Li, J.; Chen, X.; Gong, Z. X.
2015-12-01
In this work, the dynamic behavior of the non-equilibrium cavitation occurring around the underwater projectiles navigating with variable speed was numerically and theoretically investigated. The cavity collapse induced by the decelerating motion of the projectiles can be classified into two types: periodic oscillation and damped oscillation. In each type the evolution of the total mass of vapor in cavity are found to have strict correlation with the pressure oscillation in far field. By defining the equivalent radius of cavity, we introduce the specific kinetic energy of collapse and demonstrate that its change-rate is in good agreement with the pressure disturbance. We numerically investigated the influence of angle of attack on the collapse effect. The result shows that when the projectile decelerates, an asymmetric-focusing effect of the pressure induced by collapse occurs on its pressure side. We analytically explained such asymmetric-focusing effect.
Schellenberg, Barbara; Wang, Pengbo; Keeble, James A; Rodriguez-Enriquez, Ricardo; Walker, Scott; Owens, Thomas W; Foster, Fiona; Tanianis-Hughes, Jolanta; Brennan, Keith; Streuli, Charles H; Gilmore, Andrew P
2013-03-07
The proapoptotic Bcl-2 protein Bax is predominantly found in the cytosol of nonapoptotic cells and is commonly thought to translocate to mitochondria following an apoptotic stimulus. The current model for Bax activation is that BH3 proteins bind to cytosolic Bax, initiating mitochondrial targeting and outer-membrane permeabilization. Here, we challenge this and show that Bax is constitutively targeted to mitochondria but in nonapoptotic cells is constantly translocated back to the cytosol. Using live-cell spinning-disk confocal imaging with a combination of FLIP, FRAP, and photoactivatable GFP-Bax, we demonstrate that disrupting adhesion-dependent survival signals slows the rate of Bax's dissociation from mitochondria, leading to its accumulation on the outer mitochondrial membrane. The overall accumulation of mitochondrial Bax following loss of survival signaling sensitizes cells to proapoptotic BH3 proteins. Our findings show that Bax is normally in a dynamic equilibrium between cytosol and mitochondria, enabling fluctuations in survival signals to finely adjust apoptotic sensitivity.
Regime switches induced by supply-demand equilibrium: a model for power-price dynamics
NASA Astrophysics Data System (ADS)
Mari, Carlo; Tondini, Daniela
2010-11-01
Regime-switching models can be used to describe stochastic movements of electricity prices in deregulated markets. This paper shows that regime-switching dynamics arise quite naturally in an equilibrium context in which the functional form of the supply curve is described by a two-state Markov process. This mechanism is responsible for random switches between regimes and it allows one to describe the main features of the price-formation process. With the interplay between demand and supply, the proposed methodology can be used to capture shortages in electricity generation, forced outages, and peaks in electricity demand. As an example of application, a two-regime model specification is proposed, and it will be shown that the empirical analysis, performed by estimating using the model on the California power market, offers an interesting agreement with observed data.
NASA Astrophysics Data System (ADS)
Jamali, Safa; Boromand, Arman; Khani, Shaghayegh; Maia, Joao
2015-12-01
We present in this letter an auxiliary thermostat for non-equilibrium simulations in Dissipative Particle Dynamics based on the Gaussian distribution of particle velocities in the fluid. We demonstrate the ability of the thermostat to maintain the temperature under a wide range of shear rates and dissipative parameters, and to extend the shear rate window accessible by DPD significantly. The effect of proposed method on the viscosity of a DPD fluid is studied which is particularly of interest when the rheological behavior of a complex fluids is subject of DPD simulations. Furthermore, performance of the proposed method is compared to the ones from the well-known Lowe-Andersen scheme in regards to temperature and viscosity measurements.
Equilibrium dynamics of the Dean-Kawasaki equation: Mode-coupling theory and its extension
NASA Astrophysics Data System (ADS)
Kim, Bongsoo; Kawasaki, Kyozi; Jacquin, Hugo; van Wijland, Frédéric
2014-01-01
We extend a previously proposed field-theoretic self-consistent perturbation approach for the equilibrium dynamics of the Dean-Kawasaki equation presented in [Kim and Kawasaki, J. Stat. Mech. (2008) P02004, 10.1088/1742-5468/2008/02/P02004]. By taking terms missing in the latter analysis into account we arrive at a set of three new equations for correlation functions of the system. These correlations involve the density and its logarithm as local observables. Our new one-loop equations, which must carefully deal with the noninteracting Brownian gas theory, are more general than the historic mode-coupling one in that a further approximation corresponding to Gaussian density fluctuations leads back to the original mode-coupling equation for the density correlations alone. However, without performing any further approximation step, our set of three equations does not feature any ergodic-nonergodic transition, as opposed to the historical mode-coupling approach.
Equilibrium dynamics of the Dean-Kawasaki equation: mode-coupling theory and its extension.
Kim, Bongsoo; Kawasaki, Kyozi; Jacquin, Hugo; van Wijland, Frédéric
2014-01-01
We extend a previously proposed field-theoretic self-consistent perturbation approach for the equilibrium dynamics of the Dean-Kawasaki equation presented in [Kim and Kawasaki, J. Stat. Mech. (2008) P02004]. By taking terms missing in the latter analysis into account we arrive at a set of three new equations for correlation functions of the system. These correlations involve the density and its logarithm as local observables. Our new one-loop equations, which must carefully deal with the noninteracting Brownian gas theory, are more general than the historic mode-coupling one in that a further approximation corresponding to Gaussian density fluctuations leads back to the original mode-coupling equation for the density correlations alone. However, without performing any further approximation step, our set of three equations does not feature any ergodic-nonergodic transition, as opposed to the historical mode-coupling approach.
NASA Astrophysics Data System (ADS)
Sidebottom, D. L.; Tran, Tri D.
2010-11-01
Dynamic light scattering performed on aqueous solutions of three sugars (glucose, maltose and sucrose) reveal a common pattern of sugar cluster formation with a narrow cluster size distribution. In each case, equilibrium clusters form whose size increases with increasing sugar content in an identical power law manner in advance of a common, critical-like, percolation threshold near 83wt% sugar. The critical exponent of the power law divergence of the cluster size varies with temperature, increasing with decreasing temperature, due to changes in the strength of the intermolecular hydrogen bond and appears to vanish for temperatures in excess of 90°C . Detailed analysis of the cluster growth process suggests a two-stage process: an initial cluster phase formed at low volume fractions, ϕ , consisting of noninteracting, monodisperse sugar clusters whose size increases ϕ1/3 followed by an aggregation stage, active at concentrations above about ϕ=40% , where cluster-cluster contact first occurs.
Sciolla, Bruno; Biroli, Giulio
2010-11-26
We study the off-equilibrium dynamics of the infinite-dimensional Bose-Hubbard model after a quantum quench. The dynamics can be analyzed exactly by mapping it to an effective Newtonian evolution. For integer filling, we find a dynamical transition separating regimes of small and large quantum quenches starting from the superfluid state. This transition is very similar to the one found for the fermionic Hubbard model by mean field approximations.
Structure and dynamics of cylindrical micelles at equilibrium and under shear flow
NASA Astrophysics Data System (ADS)
Huang, C.-C.; Ryckaert, J.-P.; Xu, H.
2009-04-01
The dynamics and rheology of semidilute unentangled micellar solutions are investigated by Langevin dynamics mesoscopic simulations coupled to a microreversible kinetic model for scissions and recombinations. Two equilibrium state points, differing by the scission energy and therefore by the corresponding average micelle length, have been examined. The kinetic rates are tuned by an independent parameter of the model, whose range is chosen in such a way that the kinetics always strongly couple to the chain dynamics. Our results confirm, as predicted by Faivre and Gardissat, that the stress relaxation, as well as the monomer diffusion, is characterized by a time τΛ , defined by the lifetime of a segment Λ , whose Rouse relaxation time is equal to its lifetime. Moreover, the power-law dependence of the zero-shear viscosity versus τΛ was evidenced. Under stationary shear, the chains are deformed and their average bond length is increased, which enhances the overall scission frequency. In turn, this induces an overall shortening of the chains in order to increase the overall corresponding chain-end recombination frequency, as required by the stationary conditions. Nonequilibrium simulations show that the chain deformation and orientation, as well as the rheology of the system, can be expressed as universal functions of a single reduced shear rate βΛ=γ˙τΛ (with γ˙ the bare shear rate). Furthermore, local analysis of the kinetics under stationary shear gives insights on the variation of the average length with shear rate.
NASA Astrophysics Data System (ADS)
Snezhko, Alexey
2010-03-01
Ensembles of interacting particles subject to an external periodic forcing often develop nontrivial collective behavior and self-assembled dynamic patterns. We study emergent phenomena in magnetic granular ensembles suspended at a liquid-air and liquid-liquid interfaces and subjected to a transversal alternating magnetic field. Experiments reveal a new type of nontrivially ordered dynamic self-assembled structures (in particular, ``magnetic snakes'', ``asters'', ``clams'') emerging in such systems in a certain range of excitation parameters. These non-equilibrium dynamic structures emerge as a result of the competition between magnetic and hydrodynamic forces and have complex magnetic ordering. Transition between different self-assembled phases with parameters of external driving magnetic field is observed. I will show that above some frequency threshold magnetic snakes spontaneously break the symmetry of the self-induced surface flows (symmetry breaking instability) and turn into swimmers. Self-induced surface flows symmetry can be also broken in a controlled fashion by introduction of a large bead to a magnetic snake (bead-snake hybrid), that transforms it into a robust self-locomoting entity. Some features of the self-localized structures can be understood in the framework of an amplitude equation for parametric waves coupled to the conservation law equation describing the evolution of the magnetic particle density and the Navier-Stokes equation for hydrodynamic flows.
New tools for far-from-equilibrium quantum spin dynamics inspired by ultracold molecule experiments
NASA Astrophysics Data System (ADS)
Hazzard, Kaden; Foss-Feig, Michael; Gadway, Bryce; Yan, Bo; Moses, Steven; Covey, Jacob; Jin, Deborah; Ye, Jun; Rey, Ana Maria
2014-03-01
We describe new numerical techniques based on a type of cluster expansion and analytic solutions for treating far-from-equilibrium dynamics in quantum many-body spin models. Specifically, we apply them to dynamics following a quantum quench that is routinely implemented in experiments with Ramsey spectroscopy. For many observables, these new approaches converge extremely rapidly compared to existing techniques, which are unable to converge using any feasible computational resources. We describe the theoretical methods and our understanding of their superior convergence. These calculations are motivated by recent experiments with ultracold molecules in optical lattices [ Yan et al., Nature 501, 521 (2013) ] and trapped ions [ Britton et al., Nature 484, 489 (2012) ], which are described by spin models with long-range interactions in appropriate limits. We will compare theoretical predictions with experimental observations in these systems. We expect the novel methods developed to describe ultracold matter to also have applications to solid state systems, for example in the dynamics of nitrogen-vacancy centers in diamond or energy transfer in complicated molecules.
General methods for sensitivity analysis of equilibrium dynamics in patch occupancy models
Miller, David A.W.
2012-01-01
Sensitivity analysis is a useful tool for the study of ecological models that has many potential applications for patch occupancy modeling. Drawing from the rich foundation of existing methods for Markov chain models, I demonstrate new methods for sensitivity analysis of the equilibrium state dynamics of occupancy models. Estimates from three previous studies are used to illustrate the utility of the sensitivity calculations: a joint occupancy model for a prey species, its predators, and habitat used by both; occurrence dynamics from a well-known metapopulation study of three butterfly species; and Golden Eagle occupancy and reproductive dynamics. I show how to deal efficiently with multistate models and how to calculate sensitivities involving derived state variables and lower-level parameters. In addition, I extend methods to incorporate environmental variation by allowing for spatial and temporal variability in transition probabilities. The approach used here is concise and general and can fully account for environmental variability in transition parameters. The methods can be used to improve inferences in occupancy studies by quantifying the effects of underlying parameters, aiding prediction of future system states, and identifying priorities for sampling effort.
Dynamic assembly of molecularly imprinted polymer nanoparticles.
Gong, Haiyue; Hajizadeh, Solmaz; Jiang, Lingdong; Ma, Huiting; Ye, Lei
2017-09-11
Manipulation of specific binding and recycling of materials are two important aspects for practical applications of molecularly imprinted polymers. In this work, we developed a new approach to control the dynamic assembly of molecularly imprinted nanoparticles by surface functionalization. Molecularly imprinted polymer nanoparticles with a well-controlled core-shell structure were synthesized using precipitation polymerization. The specific binding sites were created in the core during the first step imprinting reaction. In the second polymerization step, epoxide groups were introduced into the particle shell to act asan intermediate linker to immobilize phenylboronic acids, as well as to introduce cis-diol structures on surface. The imprinted polymer nanoparticles modified with boronic acid and cis-diol structures maintained high molecular binding specificity, and the nanoparticles could be induced to form dynamic particle aggregation that responded to pH variation and chemical stimuli. The possibility of modulating molecular binding and nanoparticle assembly in a mutually independent fashion can be exploited in a number of applications where repeated use of precious nanoparticles is needed. Copyright © 2017 Elsevier Inc. All rights reserved.
Tunable Interfacial Thermal Conductance by Molecular Dynamics
NASA Astrophysics Data System (ADS)
Shen, Meng
We study the mechanism of tunable heat transfer through interfaces between solids using a combination of non-equilibrium molecular dynamics simulation (NEMD), vibrational mode analysis and wave packet simulation. We investigate how heat transfer through interfaces is affected by factors including pressure, interfacial modulus, contact area and interfacial layer thickness, with an overreaching goal of developing fundamental knowledge that will allow one to tailor thermal properties of interfacial materials. The role of pressure and interfacial stiffness is unraveled by our studies on an epitaxial interface between two Lennard-Jones (LJ) crystals. The interfacial stiffness is varied by two different methods: (i) indirectly by applying pressure which due to anharmonic nature of bonding, increases interfacial stiffness, and (ii) directly by changing the interfacial bonding strength by varying the depth of the potential well of the LJ potential. When the interfacial bonding strength is low, quantitatively similar behavior to pressure tuning is observed when the interfacial thermal conductance is increased by directly varying the potential-well depth parameter of the LJ potential. By contrast, when the interfacial bonding strength is high, thermal conductance is almost pressure independent, and even slightly decreases with increasing pressure. This decrease can be explained by the change in overlap between the vibrational densities of states of the two crystalline materials. The role of contact area is studied by modeling structures comprised of Van der Waals junctions between single-walled nanotubes (SWCNT). Interfacial thermal conductance between SWCNTs is obtained from NEMD simulation as a function of crossing angle. In this case the junction conductance per unit area is essentially a constant. By contrast, interfacial thermal conductance between multiwalled carbon nanotubes (MWCNTs) is shown to increase with diameter of the nanotubes by recent experimental studies [1
Dynamic strength of molecular adhesion bonds.
Evans, E; Ritchie, K
1997-04-01
In biology, molecular linkages at, within, and beneath cell interfaces arise mainly from weak noncovalent interactions. These bonds will fail under any level of pulling force if held for sufficient time. Thus, when tested with ultrasensitive force probes, we expect cohesive material strength and strength of adhesion at interfaces to be time- and loading rate-dependent properties. To examine what can be learned from measurements of bond strength, we have extended Kramers' theory for reaction kinetics in liquids to bond dissociation under force and tested the predictions by smart Monte Carlo (Brownian dynamics) simulations of bond rupture. By definition, bond strength is the force that produces the most frequent failure in repeated tests of breakage, i.e., the peak in the distribution of rupture forces. As verified by the simulations, theory shows that bond strength progresses through three dynamic regimes of loading rate. First, bond strength emerges at a critical rate of loading (> or = 0) at which spontaneous dissociation is just frequent enough to keep the distribution peak at zero force. In the slow-loading regime immediately above the critical rate, strength grows as a weak power of loading rate and reflects initial coupling of force to the bonding potential. At higher rates, there is crossover to a fast regime in which strength continues to increase as the logarithm of the loading rate over many decades independent of the type of attraction. Finally, at ultrafast loading rates approaching the domain of molecular dynamics simulations, the bonding potential is quickly overwhelmed by the rapidly increasing force, so that only naked frictional drag on the structure remains to retard separation. Hence, to expose the energy landscape that governs bond strength, molecular adhesion forces must be examined over an enormous span of time scales. However, a significant gap exists between the time domain of force measurements in the laboratory and the extremely fast scale
Jellali, S; Diamantopoulos, E; Kallali, H; Bennaceur, S; Anane, M; Jedidi, N
2010-01-01
The release of excess nitrogen-containing compounds into groundwater is a major concern in aquifer recharge by the Soil Aquifer Treatment (SAT) process. Ammonium (NH(4)(+)) is one of the most nocive and common nitrogen compounds in wastewaters. In order to assess the risk of wastewater use for aquifer recharge, NH(4)(+)adsorption onto Souhil wadi soil sampled from the SAT pilot plant (Nabeul, Tunisia) was studied using laboratory columns experiments. Several experiments were conducted using aqueous synthetic solutions under different aqueous ammonium concentrations and flow rates. Furthermore, a real wastewater solution was used to test the effect of competitive cations contents on NH(4)(+) adsorption. Afterwards, the Hydrus-1D model was used in inverse mode to simulate the ammonium transport through the Souhil wadi soil. For the synthetic solutions, the adsorbed ammonium amount varied from 1 to 30.7 mg kg(-1) for aqueous ammonium concentrations between 4.9 and 36.4 mg L(-1). The linear isotherm model was found to be the most suitable for describing this adsorption. The flow rate decrease from 45 to 15 mL min(-1) induced an increase in the ammonium adsorption capacity by 49%. Indeed, the lesser the flow rate is, the longer the residence time and the higher the exchange between the aqueous solution and soil matrix. The use of wastewater instead of aqueous synthetic solution decreased about 7 times the Souhil wadi adsorption capacity of ammonium because of its relatively high concentrations of competitive ions such as calcium and magnesium. The use of the Hydrus-1D model showed that the chemical non-equilibrium model was the best to simulate the ammonium transport through the laboratory soil columns.
Exciton dynamics in perturbed vibronic molecular aggregates
Brüning, C.; Wehner, J.; Hausner, J.; Wenzel, M.; Engel, V.
2015-01-01
A site specific perturbation of a photo-excited molecular aggregate can lead to a localization of excitonic energy. We investigate this localization dynamics for laser-prepared excited states. Changing the parameters of the electric field significantly influences the exciton localization which offers the possibility for a selective control of this process. This is demonstrated for aggregates possessing a single vibrational degree of freedom per monomer unit. It is shown that the effects identified for the molecular dimer can be generalized to larger aggregates with a high density of vibronic states. PMID:26798840
Choice of timestep in molecular dynamics simulation
NASA Astrophysics Data System (ADS)
Fincham, David
1986-06-01
In molecular dynamics computer simulation of liquids it is important to use as large a timestep as possible in order to sample phase space rapidly and save on computer expense. The effect of the resulting algorithm errors in the trajectories of the molecules is not well understood. An empirical investigation into this question is reported. Several simulations differing only in the timestep used are compared. It is found that much larger timesteps than usual can be employed without producing significant errors in observed thermodynamic, structural or dynamic properties.
Molecular dynamics on nonequilibrium motion of a colloidal particle driven by an external torque
NASA Astrophysics Data System (ADS)
Yoo, Donghwan; Jung, Youngkyun; Kwon, Chulan
2017-03-01
We investigate the motion of a colloidal particle driven out of equilibrium by an external torque. We use molecular dynamics simulation as an alternative to the Langevin dynamics. We prepare a heat bath composed of thousands of particles interacting with each other through the Lennard–Jones potential and impose the Langevin thermostat to maintain the heat bath in equilibrium. We consider a single colloidal particle interacting with with the particles of the heat bath also by the Lennard–Jones potential, without applying any types of dissipative or fluctuating forces used in Langevin dynamics. We set up simulation protocol fit for the overdamped limit as in real experiments, by increasing the size and mass of the colloidal particle. We study nonequilibrium fluctuations for work and heat produced incessantly in time and compare the results with those obtained from the previous studies via the overdamped Langevin dynamics. We confirm the Gallavotti–Cohen symmetry and the fluctuation theorem.
Multiscale Modeling using Molecular Dynamics and Dual Domain Material Point Method
Dhakal, Tilak Raj
2016-07-07
For problems involving large material deformation rate, the material deformation time scale can be shorter than the material takes to reach a thermodynamical equilibrium. For such problems, it is difficult to obtain a constitutive relation. History dependency become important because of thermodynamic non-equilibrium. Our goal is to build a multi-scale numerical method which can bypass the need for a constitutive relation. In conclusion, multi-scale simulation method is developed based on the dual domain material point (DDMP). Molecular dynamics (MD) simulation is performed to calculate stress. Since the communication among material points is not necessary, the computation can be done embarrassingly parallel in CPU-GPU platform.
Kreula, J. M.; Clark, S. R.; Jaksch, D.
2016-01-01
We propose a non-linear, hybrid quantum-classical scheme for simulating non-equilibrium dynamics of strongly correlated fermions described by the Hubbard model in a Bethe lattice in the thermodynamic limit. Our scheme implements non-equilibrium dynamical mean field theory (DMFT) and uses a digital quantum simulator to solve a quantum impurity problem whose parameters are iterated to self-consistency via a classically computed feedback loop where quantum gate errors can be partly accounted for. We analyse the performance of the scheme in an example case. PMID:27609673
Reactive Molecular Dynamics Studies of Thermal Induced Chemistry in TATB
NASA Astrophysics Data System (ADS)
Germann, Timothy; Quenneville, Jason
2007-03-01
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers [1] at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 800-particle simulations at several temperatures, and detail current capabilities for large-scale (10^4 -- 10^5 atoms) systems carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reaction timescales with those of RDX and HMX. [1] A. C. T. Van Duin, et al, J. Phys. Chem. A, 1005, 9396 (2001).
Molecular Dynamics Simulations of Thermal Induced Chemistry in TATB
NASA Astrophysics Data System (ADS)
Quenneville, Jason; Germann, Timothy
2006-03-01
Equilibrium molecular dynamics (MD) simulation of high explosives can provide important information on their thermal decomposition by helping to characterize processes with timescales that are much longer than those attainable with non-equilibrium MD shock studies. A reactive force field is used with MD to probe the chemisty induced by intense heating (`cook-off') of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). The force field (ReaxFF) was developed by van Duin, Goddard and coworkers^ at CalTech and has already shown promise in predicting the chemistry in small samples of RDX under either shock compression or intense heat. Large-system simulations are desired for TATB because of the high degree of carbon clustering expected in this material. We will show results of 100,000-particle simulations at several temperatures, carried out with the massively parallel GRASP MD software developed at Sandia National Lab. Finally, we will compare the reactions and reaction timescales with those of RDX and HMX. ^ A. C. T. Van Duin, et al, J. Phys. Chem. A, 1005, 9396 (2001).
Stresses and elastic constants of crystalline sodium, from molecular dynamics
Schiferl, S.K.
1985-02-01
The stresses and the elastic constants of bcc sodium are calculated by molecular dynamics (MD) for temperatures to T = 340K. The total adiabatic potential of a system of sodium atoms is represented by pseudopotential model. The resulting expression has two terms: a large, strictly volume-dependent potential, plus a sum over ion pairs of a small, volume-dependent two-body potential. The stresses and the elastic constants are given as strain derivatives of the Helmholtz free energy. The resulting expressions involve canonical ensemble averages (and fluctuation averages) of the position and volume derivatives of the potential. An ensemble correction relates the results to MD equilibrium averages. Evaluation of the potential and its derivatives requires the calculation of integrals with infinite upper limits of integration, and integrand singularities. Methods for calculating these integrals and estimating the effects of integration errors are developed. A method is given for choosing initial conditions that relax quickly to a desired equilibrium state. Statistical methods developed earlier for MD data are extended to evaluate uncertainties in fluctuation averages, and to test for symmetry. 45 refs., 10 figs., 4 tabs.
Molecular Dynamics of a Protein Surface: Ion-Residues Interactions
Friedman, Ran; Nachliel, Esther; Gutman, Menachem
2005-01-01
Time-resolved measurements indicated that protons could propagate on the surface of a protein or a membrane by a special mechanism that enhanced the shuttle of the proton toward a specific site. It was proposed that a suitable location of residues on the surface contributes to the proton shuttling function. In this study, this notion was further investigated by the use of molecular dynamics simulations, where Na+ and Cl− are the ions under study, thus avoiding the necessity for quantum mechanical calculations. Molecular dynamics simulations were carried out using as a model a few Na+ and Cl− ions enclosed in a fully hydrated simulation box with a small globular protein (the S6 of the bacterial ribosome). Three independent 10-ns-long simulations indicated that the ions and the protein's surface were in equilibrium, with rapid passage of the ions between the protein's surface and the bulk. However, it was noted that close to some domains the ions extended their duration near the surface, thus suggesting that the local electrostatic potential hindered their diffusion to the bulk. During the time frame in which the ions were detained next to the surface, they could rapidly shuttle between various attractor sites located under the electrostatic umbrella. Statistical analysis of the molecular dynamics and electrostatic potential/entropy consideration indicated that the detainment state is an energetic compromise between attractive forces and entropy of dilution. The similarity between the motion of free ions next to a protein and the proton transfer on the protein's surface are discussed. PMID:15894639
Molecular dynamics at constant Cauchy stress.
Miller, Ronald E; Tadmor, Ellad B; Gibson, Joshua S; Bernstein, Noam; Pavia, Fabio
2016-05-14
The Parrinello-Rahman algorithm for imposing a general state of stress in periodic molecular dynamics simulations is widely used in the literature and has been implemented in many readily available molecular dynamics codes. However, what is often overlooked is that this algorithm controls the second Piola-Kirchhoff stress as opposed to the true (Cauchy) stress. This can lead to misinterpretation of simulation results because (1) the true stress that is imposed during the simulation depends on the deformation of the periodic cell, (2) the true stress is potentially very different from the imposed second Piola-Kirchhoff stress, and (3) the true stress can vary significantly during the simulation even if the imposed second Piola-Kirchhoff is constant. We propose a simple modification to the algorithm that allows the true Cauchy stress to be controlled directly. We then demonstrate the efficacy of the new algorithm with the example of martensitic phase transformations under applied stress.
Molecular dynamics studies of polyurethane nanocomposite hydrogels
NASA Astrophysics Data System (ADS)
Strankowska, J.; Piszczyk, Ł.; Strankowski, M.; Danowska, M.; Szutkowski, K.; Jurga, S.; Kwela, J.
2013-10-01
Polyurethane PEO-based hydrogels have a broad range of biomedical applicability. They are attractive for drug-controlled delivery systems, surgical implants and wound healing dressings. In this study, a PEO based polyurethane hydrogels containing Cloisite® 30B, an organically modified clay mineral, was synthesized. Structure of nanocomposite hydrogels was determined using XRD technique. Its molecular dynamics was studied by means of NMR spectroscopy, DMA and DSC analysis. The mechanical properties and thermal stability of the systems were improved by incorporation of clay and controlled by varying the clay content in polymeric matrix. Molecular dynamics of polymer chains depends on interaction of Cloisite® 30B nanoparticles with soft segments of polyurethanes. The characteristic nanosize effect is observed.
NASA Astrophysics Data System (ADS)
Benecke, A.
2006-03-01
Chromatin is a, if not the, hallmark of eukaryotic life. Any molecular process entailing genomic DNA or the nucleus by default provokes or depends on chromatin structural dynamics on various space and time scales. Chromatin dynamics are result of changes in the physico-chemical properties of the chromatin constituents themselves or the nuclear environment. Chromatin has been found in the former case to undergo many different covalent enzyme-mediated chemical modifications. Their identification sheds light on the molecular mechanisms and the physico-chemical properties underlying chromatin dynamics, and allows the development of quantitative models for the chromatin fiber. The abundance of the different modifications, their dynamics, and short- as well as long-range correlation phenomena between different modifications also point to a second layer of genomic coding implemented at the level of chromatin. Especially, gene regulatory coding seems to depend on such a second-level code. The information-theoretical properties of chromatin in the context of gene regulatory coding are discussed. A model for the emergence of cellular differentiation from the intricate interplay between genomic and chromatin code is presented and discussed in light of recent experimental insights.
Molecular dynamics simulations of high speed rarefied gas flows
NASA Astrophysics Data System (ADS)
Dongari, Nishanth; Zhang, Yonghao; Reese, Jason M.
2012-11-01
To understand the molecular behaviour of gases in high speed rarefied conditions, we perform molecular dynamics (MD) numerical experiments using the open source code Open FOAM. We use shear-driven Couette flows as test cases, where the two parallel plates are moving with a speed of Uw in opposite directions with their temperatures set to Tw. The gas rarefaction conditions vary from slip to transition, and compressibility conditions vary from low speed isothermal to hypersonic flow regimes, i.e. Knudsen number (Kn) from 0.01 to 1 and Mach number (Ma) from 0.05 to 10. We measure the molecular velocity distribution functions, the spatial variation of gas mean free path profiles and other macroscopic properties. Our MD results convey that flow properties in the near-wall non-equilibrium region do not merely depend on Kn, but they are also significantly affected by Ma. These results may yield new insight into diffusive transport in rarefied gases at high speeds.
Molecular dynamics modelling of solidification in metals
Boercker, D.B.; Belak, J.; Glosli, J.
1997-12-31
Molecular dynamics modeling is used to study the solidification of metals at high pressure and temperature. Constant pressure MD is applied to a simulation cell initially filled with both solid and molten metal. The solid/liquid interface is tracked as a function of time, and the data are used to estimate growth rates of crystallites at high pressure and temperature in Ta and Mg.
Vacuum Ultraviolet Studies of Molecular Dynamics
1992-01-15
the Journal of Chemical Physics . Vacuum Ultraviolet Studies of Molecular Dynamics Page 4 B. Quenching of S(’D) by N2...An article on this work has been published in the Journal of Chemical Physics . E. The 157 am Photodissoclation of OCS The photodissociation of OCS...angular momentum vectors are perpendicular to one another. A report of this work has been published in the Journal of Chemical Physics . Vacuum
New faster CHARMM molecular dynamics engine
Hynninen, Antti-Pekka; Crowley, Michael F
2014-01-01
We introduce a new faster molecular dynamics (MD) engine into the CHARMM software package. The new MD engine is faster both in serial (i.e., single CPU core) and parallel execution. Serial performance is approximately two times higher than in the previous version of CHARMM. The newly programmed parallelization method allows the MD engine to parallelize up to hundreds of CPU cores. PMID:24302199
Molecular crowding and protein enzymatic dynamics.
Echeverria, Carlos; Kapral, Raymond
2012-05-21
The effects of molecular crowding on the enzymatic conformational dynamics and transport properties of adenylate kinase are investigated. This tridomain protein undergoes large scale hinge motions in the course of its enzymatic cycle and serves as prototype for the study of crowding effects on the cyclic conformational dynamics of proteins. The study is carried out at a mesoscopic level where both the protein and the solvent in which it is dissolved are treated in a coarse grained fashion. The amino acid residues in the protein are represented by a network of beads and the solvent dynamics is described by multiparticle collision dynamics that includes effects due to hydrodynamic interactions. The system is crowded by a stationary random array of hard spherical objects. Protein enzymatic dynamics is investigated as a function of the obstacle volume fraction and size. In addition, for comparison, results are presented for a modification of the dynamics that suppresses hydrodynamic interactions. Consistent with expectations, simulations of the dynamics show that the protein prefers a closed conformation for high volume fractions. This effect becomes more pronounced as the obstacle radius decreases for a given volume fraction since the average void size in the obstacle array is smaller for smaller radii. At high volume fractions for small obstacle radii, the average enzymatic cycle time and characteristic times of internal conformational motions of the protein deviate substantially from their values in solution or in systems with small density of obstacles. The transport properties of the protein are strongly affected by molecular crowding. Diffusive motion adopts a subdiffusive character and the effective diffusion coefficients can change by more than an order of magnitude. The orientational relaxation time of the protein is also significantly altered by crowding.
NASA Astrophysics Data System (ADS)
Bustingorry, Sebastian; Cugliandolo, Leticia F.; Domínguez, Daniel
2007-01-01
We study the relaxation dynamics of flux lines in dirty high-temperature superconductors using numerical simulations of a London-Langevin model of the interacting vortex lines. By analyzing the equilibrium dynamics in the vortex liquid phase we find a dynamic crossover to a glassy nonequilibrium regime. We then focus on the out-of-equilibrium dynamics of the vortex glass phase using tools that are common in the study of other glassy systems. By monitoring the two-times roughness and dynamic wandering we identify and characterize finite-size effects that are similar, though more complex, than the ones found in the stationary roughness of clean interface dynamics. The two-times density-density correlation and mean-squared-displacement correlation age and their temporal scaling follows a multiplicative law similar to the one found at criticality. The linear responses also age and the comparison with their associated correlations shows that the equilibrium fluctuation-dissipation relation is modified in a simple manner that allows for the identification of an effective temperature characterizing the dynamics of the slow modes. The effective temperature is closely related to the vortex liquid-vortex glass crossover temperature. Interestingly enough, our study demonstrates that the glassy dynamics in the vortex glass is basically identical to the one of a single elastic line in a disordered environment (with the same type of scaling though with different parameters). Possible extensions and the experimental relevance of these results are also discussed.
Contact angles from Young's equation in molecular dynamics simulations
NASA Astrophysics Data System (ADS)
Jiang, Hao; Müller-Plathe, Florian; Panagiotopoulos, Athanassios Z.
2017-08-01
We propose a method to calculate the equilibrium contact angle of heterogeneous 3-phase solid/fluid/fluid systems using molecular dynamics simulations. The proposed method, which combines the phantom-wall method [F. Leroy and F. Müller-Plathe, J. Chem. Phys. 133, 044110 (2010)] and Bennett's acceptance ratio approach [C. H. Bennett, J. Comput. Phys. 22, 245 (1976)], is able to calculate the solid/fluid surface tension relative to the solid surface energy. The calculated relative surface tensions can then be used in Young's equation to estimate the equilibrium contact angle. A fluid droplet is not needed for the proposed method, in contrast to the situation for direct simulations of contact angles. In addition, while prior free-energy based methods for contact angles mainly focused on the wetting of fluids in coexistence with their vapor on solid surfaces, the proposed approach was designed to study the contact angles of fluid mixtures on solid surfaces above the fluid saturation pressures. Using the proposed approach, the contact angles of binary Lennard-Jones fluid mixtures on a non-polar solid substrate were calculated at various interaction parameters and the contact angle of water in equilibrium with CO2 on a hydrophilic polar silica surface was obtained. For both non-polar and polar systems, the calculated contact angles from the proposed method were in agreement with those obtained from the geometry of a cylindrical droplet. The computational cost of the proposed method was found to be comparable to that of simulations that use fluid droplets, but the new method provides a way to calculate the contact angle directly from Young's equation without ambiguity.
Contact angles from Young's equation in molecular dynamics simulations.
Jiang, Hao; Müller-Plathe, Florian; Panagiotopoulos, Athanassios Z
2017-08-28
We propose a method to calculate the equilibrium contact angle of heterogeneous 3-phase solid/fluid/fluid systems using molecular dynamics simulations. The proposed method, which combines the phantom-wall method [F. Leroy and F. Müller-Plathe, J. Chem. Phys. 133, 044110 (2010)] and Bennett's acceptance ratio approach [C. H. Bennett, J. Comput. Phys. 22, 245 (1976)], is able to calculate the solid/fluid surface tension relative to the solid surface energy. The calculated relative surface tensions can then be used in Young's equation to estimate the equilibrium contact angle. A fluid droplet is not needed for the proposed method, in contrast to the situation for direct simulations of contact angles. In addition, while prior free-energy based methods for contact angles mainly focused on the wetting of fluids in coexistence with their vapor on solid surfaces, the proposed approach was designed to study the contact angles of fluid mixtures on solid surfaces above the fluid saturation pressures. Using the proposed approach, the contact angles of binary Lennard-Jones fluid mixtures on a non-polar solid substrate were calculated at various interaction parameters and the contact angle of water in equilibrium with CO2 on a hydrophilic polar silica surface was obtained. For both non-polar and polar systems, the calculated contact angles from the proposed method were in agreement with those obtained from the geometry of a cylindrical droplet. The computational cost of the proposed method was found to be comparable to that of simulations that use fluid droplets, but the new method provides a way to calculate the contact angle directly from Young's equation without ambiguity.
Bead-Fourier path integral molecular dynamics.
Ivanov, Sergei D; Lyubartsev, Alexander P; Laaksonen, Aatto
2003-06-01
Molecular dynamics formulation of Bead-Fourier path integral method for simulation of quantum systems at finite temperatures is presented. Within this scheme, both the bead coordinates and Fourier coefficients, defining the path representing the quantum particle, are treated as generalized coordinates with corresponding generalized momenta and masses. Introduction of the Fourier harmonics together with the center-of-mass thermostating scheme is shown to remove the ergodicity problem, known to pose serious difficulties in standard path integral molecular dynamics simulations. The method is tested for quantum harmonic oscillator and hydrogen atom (Coulombic potential). The simulation results are compared with the exact analytical solutions available for both these systems. Convergence of the results with respect to the number of beads and Fourier harmonics is analyzed. It was shown that addition of a few Fourier harmonics already improves the simulation results substantially, even for a relatively small number of beads. The proposed Bead-Fourier path integral molecular dynamics is a reliable and efficient alternative to simulations of quantum systems.
Monoamine transporters: insights from molecular dynamics simulations
Grouleff, Julie; Ladefoged, Lucy Kate; Koldsø, Heidi; Schiøtt, Birgit
2015-01-01
The human monoamine transporters (MATs) facilitate the reuptake of the neurotransmitters serotonin, dopamine, and norepinephrine from the synaptic cleft. Imbalance in monoaminergic neurotransmission is linked to various diseases including major depression, attention deficit hyperactivity disorder, schizophrenia, and Parkinson’s disease. Inhibition of the MATs is thus an important strategy for treatment of such diseases. The MATs are sodium-coupled transport proteins belonging to the neurotransmitter/Na+ symporter (NSS) family, and the publication of the first high-resolution structure of a NSS family member, the bacterial leucine transporter LeuT, in 2005, proved to be a major stepping stone for understanding this family of transporters. Structural data allows for the use of computational methods to study the MATs, which in turn has led to a number of important discoveries. The process of substrate translocation across the membrane is an intrinsically dynamic process. Molecular dynamics simulations, which can provide atomistic details of molecular motion on ns to ms timescales, are therefore well-suited for studying transport processes. In this review, we outline how molecular dynamics simulations have provided insight into the large scale motions associated with transport of the neurotransmitters, as well as the presence of external and internal gates, the coupling between ion and substrate transport, and differences in the conformational changes induced by substrates and inhibitors. PMID:26528185
Equipartition Principle for Internal Coordinate Molecular Dynamics.
Jain, Abhinandan; Park, In-Hee; Vaidehi, Nagarajan
2012-08-14
The principle of equipartition of (kinetic) energy for all-atom Cartesian molecular dynamics states that each momentum phase space coordinate on the average has ½kT of kinetic energy in a canonical ensemble. This principle is used in molecular dynamics simulations to initialize velocities, and to calculate statistical properties such as entropy. Internal coordinate molecular dynamics (ICMD) models differ from Cartesian models in that the overall kinetic energy depends on the generalized coordinates and includes cross-terms. Due to this coupled structure, no such equipartition principle holds for ICMD models. In this paper we introduce non-canonical modal coordinates to recover some of the structural simplicity of Cartesian models and develop a new equipartition principle for ICMD models. We derive low-order recursive computational algorithms for transforming between the modal and physical coordinates. The equipartition principle in modal coordinates provides a rigorous method for initializing velocities in ICMD simulations thus replacing the ad hoc methods used until now. It also sets the basis for calculating conformational entropy using internal coordinates.
Bead-Fourier path integral molecular dynamics
NASA Astrophysics Data System (ADS)
Ivanov, Sergei D.; Lyubartsev, Alexander P.; Laaksonen, Aatto
2003-06-01
Molecular dynamics formulation of Bead-Fourier path integral method for simulation of quantum systems at finite temperatures is presented. Within this scheme, both the bead coordinates and Fourier coefficients, defining the path representing the quantum particle, are treated as generalized coordinates with corresponding generalized momenta and masses. Introduction of the Fourier harmonics together with the center-of-mass thermostating scheme is shown to remove the ergodicity problem, known to pose serious difficulties in standard path integral molecular dynamics simulations. The method is tested for quantum harmonic oscillator and hydrogen atom (Coulombic potential). The simulation results are compared with the exact analytical solutions available for both these systems. Convergence of the results with respect to the number of beads and Fourier harmonics is analyzed. It was shown that addition of a few Fourier harmonics already improves the simulation results substantially, even for a relatively small number of beads. The proposed Bead-Fourier path integral molecular dynamics is a reliable and efficient alternative to simulations of quantum systems.
Learning generative models of molecular dynamics
2012-01-01
We introduce three algorithms for learning generative models of molecular structures from molecular dynamics simulations. The first algorithm learns a Bayesian-optimal undirected probabilistic model over user-specified covariates (e.g., fluctuations, distances, angles, etc). L1 reg-ularization is used to ensure sparse models and thus reduce the risk of over-fitting the data. The topology of the resulting model reveals important couplings between different parts of the protein, thus aiding in the analysis of molecular motions. The generative nature of the model makes it well-suited to making predictions about the global effects of local structural changes (e.g., the binding of an allosteric regulator). Additionally, the model can be used to sample new conformations. The second algorithm learns a time-varying graphical model where the topology and parameters change smoothly along the trajectory, revealing the conformational sub-states. The last algorithm learns a Markov Chain over undirected graphical models which can be used to study and simulate kinetics. We demonstrate our algorithms on multiple molecular dynamics trajectories. PMID:22369071
Learning generative models of molecular dynamics.
Razavian, Narges Sharif; Kamisetty, Hetunandan; Langmead, Christopher J
2012-01-01
We introduce three algorithms for learning generative models of molecular structures from molecular dynamics simulations. The first algorithm learns a Bayesian-optimal undirected probabilistic model over user-specified covariates (e.g., fluctuations, distances, angles, etc). L1 regularization is used to ensure sparse models and thus reduce the risk of over-fitting the data. The topology of the resulting model reveals important couplings between different parts of the protein, thus aiding in the analysis of molecular motions. The generative nature of the model makes it well-suited to making predictions about the global effects of local structural changes (e.g., the binding of an allosteric regulator). Additionally, the model can be used to sample new conformations. The second algorithm learns a time-varying graphical model where the topology and parameters change smoothly along the trajectory, revealing the conformational sub-states. The last algorithm learns a Markov Chain over undirected graphical models which can be used to study and simulate kinetics. We demonstrate our algorithms on multiple molecular dynamics trajectories.
Fragment Molecular Orbital Nonadiabatic Molecular Dynamics for Condensed Phase Systems.
Nebgen, Ben; Prezhdo, Oleg V
2016-09-15
A method for efficiently simulating nonadiabatic molecular dynamics (NAMD) of nanoscale and condensed phase systems is developed and tested. The electronic structure, including force and nonadiabatic coupling, are obtained with the fragment molecular orbital (FMO) approximation, which provides significant computational savings by splitting the system into fragments and computing electronic properties of each fragment subject to the external field due to other all other fragments. The efficiency of the developed technique is demonstrated by studying the effect of explicit solvent molecules on excited state relaxation in the Fe(CO)4 complex. The relaxation in the gas phase occurs on a 50 fs time scale, which is in excellent agreement with previously recorded femtosecond pump-probe spectroscopy. Adding a solvation shell of ethanol molecules to the simulation results in an increase in the excited state lifetime to 100 fs, in agreement with recent femtosecond X-ray spectroscopy measurements.
Eisosomes Are Dynamic Plasma Membrane Domains Showing Pil1-Lsp1 Heteroligomer Binding Equilibrium
Olivera-Couto, Agustina; Salzman, Valentina; Mailhos, Milagros; Digman, Michelle A.; Gratton, Enrico; Aguilar, Pablo S.
2015-01-01
Eisosomes are plasma membrane domains concentrating lipids, transporters, and signaling molecules. In the budding yeast Saccharomyces cerevisiae, these domains are structured by scaffolds composed mainly by two cytoplasmic proteins Pil1 and Lsp1. Eisosomes are immobile domains, have relatively uniform size, and encompass thousands of units of the core proteins Pil1 and Lsp1. In this work we used fluorescence fluctuation analytical methods to determine the dynamics of eisosome core proteins at different subcellular locations. Using a combination of scanning techniques with autocorrelation analysis, we show that Pil1 and Lsp1 cytoplasmic pools freely diffuse whereas an eisosome-associated fraction of these proteins exhibits slow dynamics that fit with a binding-unbinding equilibrium. Number and brightness analysis shows that the eisosome-associated fraction is oligomeric, while cytoplasmic pools have lower aggregation states. Fluorescence lifetime imaging results indicate that Pil1 and Lsp1 directly interact in the cytoplasm and within the eisosomes. These results support a model where Pil1-Lsp1 heterodimers are the minimal eisosomes building blocks. Moreover, individual-eisosome fluorescence fluctuation analysis shows that eisosomes in the same cell are not equal domains: while roughly half of them are mostly static, the other half is actively exchanging core protein subunits. PMID:25863055
NASA Astrophysics Data System (ADS)
Shogin, Dmitry; Amund Amundsen, Per
2016-10-01
We test the physical relevance of the full and the truncated versions of the Israel-Stewart (IS) theory of irreversible thermodynamics in a cosmological setting. Using a dynamical systems method, we determine the asymptotic future of plane symmetric Bianchi type I spacetimes with a viscous mathematical fluid, keeping track of the magnitude of the relative dissipative fluxes, which determines the applicability of the IS theory. We consider the situations where the dissipative mechanisms of shear and bulk viscosity are involved separately and simultaneously. It is demonstrated that the only case in the given model when the fluid asymptotically approaches local thermal equilibrium, and the underlying assumptions of the IS theory are therefore not violated, is that of a dissipative fluid with vanishing bulk viscosity. The truncated IS equations for shear viscosity are found to produce solutions which manifest pathological dynamical features and, in addition, to be strongly sensitive to the choice of initial conditions. Since these features are observed already in the case of an oversimplified mathematical fluid model, we have no reason to assume that the truncation of the IS transport equations will produce relevant results for physically more realistic fluids. The possible role of bulk and shear viscosity in cosmological evolution is also discussed.
Saltmarsh pool and tidal creek morphodynamics: Dynamic equilibrium of northern latitude saltmarshes?
NASA Astrophysics Data System (ADS)
Wilson, Carol A.; Hughes, Zoe J.; FitzGerald, Duncan M.; Hopkinson, Charles S.; Valentine, Vinton; Kolker, Alexander S.
2014-05-01
Many saltmarsh platforms in New England and other northern climates (e.g. Canada, northern Europe) exhibit poor drainage, creating waterlogged regions where short-form Spartina alterniflora dominates and stagnant pools that experience tidal exchange only during spring tides and storm-induced flooding events. The processes related to pool formation and tidal creek incision (via headward erosion) that may eventually drain these features are poorly understood, however it has been suggested that an increase in pool occurrence in recent decades is due to waterlogging stress from sea-level rise. We present evidence here that saltmarshes in Plum Island Estuary of Massachusetts are keeping pace with sea-level rise, and that the recent increase in saltmarsh pool area coincides with changes in drainage density from a legacy of anthropogenic ditching (reversion to natural drainage conditions). Gradients, in addition to elevation and hydroperiod, are critical for saltmarsh pool formation. Additionally, elevation and vegetative changes associated with pool formation, creek incision, subsequent drainage of pools, and recolonization by S. alterniflora are quantified. Pool and creek dynamics were found to be cyclic in nature, and represent platform elevation in dynamic equilibrium with sea level whereby saltmarsh elevation may be lowered (due to degradation of organic matter and formation of a pool), however may be regained on short timescales (101-2 yr) with creek incision into pools and restoration of tidal exchange. Rapid vertical accretion is associated with sedimentation and S. alterniflora plant recolonization.
Freckleton, R P; Watkinson, A R; Dowling, P M; Ley, A R
2000-06-07
The dynamics of an annual pasture community are described from a five-year experimental and monitoring study. The community was dominated by two grasses (Lolium rigidum and Vulpia bromoides) and a legume (Trifolium subterraneum). Fits of population dynamic models to per capita rates of population change indicate that interactions between the grasses were generally strong, while interactions between the grasses and legumes were weaker. Most, but not all, of the net effects of competition on population growth could be attributed to interactions occurring during plant growth. Phase-plane analysis indicated that, for a constant environment, a joint equilibrium of the two grasses is unstable since interspecific competition between Lolium and Vulpia is stronger than intraspecific competition. Consequently, the community will tend to a mixture of only one or other of the grass species and T. subterraneum, depending on the founding composition of the pasture. Analysis of data taken from a year in which a drought occurred (1993-1994) demonstrated profound effects on all three species. Modelling of the long-term impacts of the effects of repeated droughts showed that disturbance of this form overrides the founder effect observed under constant conditions. Consequently, Vulpia is ultimately able to invade any mixture of the other species in environments where stochastic disturbances occur.
The deceiving Δ′: On the equilibrium dependent dynamics of nonlinear magnetic islands
Militello, F.; Grasso, D.; Borgogno, D.
2014-10-15
The linear stability parameter Δ′ is commonly used as a figure of merit for the nonlinear dynamics of the tearing mode. It is shown, through numerical simulations, that factors other than Δ′ can play a very important role in determining the evolution of nonlinear magnetic islands, even relatively close to marginal stability. In particular, two different equilibria are analysed and it is shown that, once perturbed, they have a qualitatively and quantitatively different response despite the fact that they are characterised by the same Δ′. However, the different behaviour can still be associated with linear properties of the equilibrium. It is also studied how the nonlinear and saturation phase are affected by an increasing Δ′ in the two equilibria. As the instability drive is increased, the systems move from a dynamics characterised by a “universal” generalised Rutherford equation to a Y-point configuration and then to a plasmoid unstable Y-point. Remarkably, in certain configurations the Rutherford phase is absent and the system forms a current ribbon without an X-point collapse.
Ultrafast non-equilibrium carrier dynamics in semiconductor laser mode-locking
NASA Astrophysics Data System (ADS)
Hader, J.; Scheller, M.; Laurain, A.; Kilen, I.; Baker, C.; Moloney, J. V.; Koch, S. W.
2017-01-01
Experimental and theoretical results on the mode-locking dynamics in vertical-external-cavity surface-emitting lasers with semiconductor and graphene saturable absorber mirrors are reviewed with an emphasis on the role of nonequilibrium carrier effects. The systems are studied theoretically using a fully microscopic many-body model for the carrier distributions and polarizations, coupled to Maxwell’s equations for the field propagation. Pump-probe measurements are performed with (sub-) 100 fs resolution. The analysis shows that the non-equilibrium carrier dynamics in the gain quantum-wells and saturable absorber medium significantly influences the system’s response and the resulting mode-locked pulses. The microscopic model is used to study the pulse build up from spontaneous emission noise and to determine the dependence of achievable pulse lengths and fluences on the amounts of saturable and non-saturable losses and the optical gain. The change of the group delay dispersion (GDD) on the pump level is examined and the dependence of the pulse lengths on the total amount of GDD is demonstrated experimentally. Theory-experiment comparisons are used to demonstrate the highly quantitative accuracy of the fully microscopic modeling.
Dynamics to Equilibrium in Network Games: Individual Behavior and Global Response
Cimini, Giulio; Castellano, Claudio; Sánchez, Angel
2015-01-01
Various social contexts can be depicted as games of strategic interactions on networks, where an individual’s welfare depends on both her and her partners’ actions. Whereas much attention has been devoted to Bayes-Nash equilibria in such games, here we look at strategic interactions from an evolutionary perspective. To this end, we present the results of a numerical simulations program for these games, which allows us to find out whether Nash equilibria are accessible by adaptation of player strategies, and in general to identify the attractors of the evolution. Simulations allow us to go beyond a global characterization of the cooperativeness at equilibrium and probe into individual behavior. We find that when players imitate each other, evolution does not reach Nash equilibria and, worse, leads to very unfavorable states in terms of welfare. On the contrary, when players update their behavior rationally, they self-organize into a rich variety of Nash equilibria, where individual behavior and payoffs are shaped by the nature of the game, the social network’s structure and the players’ position within the network. Our results allow to assess the validity of mean-field approaches we use to describe the dynamics of these games. Interestingly, our dynamically-found equilibria generally do not coincide with (but show qualitatively the same features of) those resulting from theoretical predictions in the context of one-shot games under incomplete information. PMID:25803275
Dynamic signaling cascades: reversible covalent reaction-coupled molecular switches.
Ren, Yulong; You, Lei
2015-11-11
The research of systems chemistry exploring complex mixtures of interacting synthetic molecules has been burgeoning recently. Herein we demonstrate for the first time the coupling of molecular switches with a dynamic covalent reaction (DCR) and the modulation of created chemical cascades with a variety of inputs, thus closely mimicking a biological signaling system. A novel Michael type DCR of 10-methylacridinium perchlorate and monothiols exhibiting excellent regioselectivity and tunable affinity was discovered. A delicate balance between the unique reactivity of the reactant and the stability of the adduct leads to the generation of a strong acid in a thermodynamically controlled system. The dynamic cascade was next created via coupling of the DCR and a protonation-induced configurational switch (E/Z isomerization) through a proton relay. Detailed examination of the interdependence of the equilibrium enabled us to rationally optimize the cascade and also shed light on the possible intermediate of the switching process. Furthermore, relative independence of the coupled reactions was verified by the identification of stimuli that are able to facilitate one reaction but suppress the other. To further enhance systematic complexity, a second DCR of electrophilic aldehydes and thiols was employed for the reversible inhibition of the binary system, thus achieving the interplay of multiple equilibria. Finally, a fluorescence switch was turned on through coupling with the DCR, showcasing the versatility of our strategy. The results described herein should pave the way for the exploitation of multifunctional dynamic covalent cascades.
Molecular Dynamics Studies of Caspase-3
Sulpizi, M.; Rothlisberger, U.; Carloni, P.
2003-01-01
Caspase-3 is a fundamental target for pharmaceutical interventions against a variety of diseases involving disregulated apoptosis. The enzyme is active as a dimer with two symmetry-related active sites, each featuring a Cys-His catalytic dyad and a selectivity loop, which recognizes the characteristic DEVD pattern of the substrate. Here, a molecular dynamics study of the enzyme in complex with two pentapeptide substrates DEVDG is presented, which provides a characterization of the dynamic properties of the active form in aqueous solution. The mobility of the substrate and that of the catalytic residues are rather low indicating a distinct preorganization effect of the Michaelis complex. An essential mode analysis permits us to identify coupled motions between the two monomers. In particular, it is found that the motions of the two active site loops are correlated and tend to steer the substrate toward the reactive center, suggesting that dimerization has a distinct effect on the dynamic properties of the active site regions. The selectivity loop of one monomer turns out to be correlated with the N-terminal region of the p12 subunit of the other monomer, an interaction that is also found to play a fundamental role in the electrostatic stabilization of the quaternary structure. To further characterize the specific influence of dimerization on the enzyme essential motions, a molecular dynamics analysis is also performed on the isolated monomer. PMID:12668429
Molecular dynamics simulations of magnetized dusty plasmas
NASA Astrophysics Data System (ADS)
Piel, Alexander; Reichstein, Torben; Wilms, Jochen
2012-10-01
The combination of the electric field that confines a dust cloud with a static magnetic field generally leads to a rotation of the dust cloud. In weak magnetic fields, the Hall component of the ion flow exerts a drag force that sets the dust in rotation. We have performed detailed molecular-dynamics simulations of the dynamics of torus-shaped dust clouds in anodic plasmas. The stationary flow [1] is characterized by a shell structure in the laminar dust flow and by the spontaneous formation of a shear-flow around a stationary vortex. Here we present new results on dynamic phenomena, among them fluctuations due to a Kelvin-Helmholtz instability in the shear-flow. The simulations are compared with experimental results. [4pt] [1] T. Reichstein, A. Piel, Phys. Plasmas 18, 083705 (2011)
NASA Astrophysics Data System (ADS)
Oprisan, Ana; Leilani Payne, Alexis
2013-03-01
Colloids are made of particles of sizes from 1 to 1000 nm dispersed in liquid, or a gas phase dispersed in a liquid (foam), or a solid or liquid dispersed in a gas (fume and fog). Colloids are ideal systems for investigating both spatial and temporal processes using optical methods since they have particle sizes larger than the characteristic size of atomic or molecular systems. We performed small angle light scattering (SALS) experiments in order to investigate the crossover between viscosity and concentration-driven non-equilibrium fluctuations. Our SALS experimental setup involved a free diffusion cell filled with nanoparticle colloids with the concentration gradient oriented against the gravitational field and a low coherence superluminescent diode (SLD) instead of a laser as the light source. By appropriately designing the optics, the speckle size of the CCD sensor can be matched with the pixel size, which makes the CCD a large ensemble of intensity autocorrelators working in parallel. Nonequilibrium concentration-driven fluctuations in three different nanocolloidal suspensions (gold, silver, and silica) with a wide range of particle sizes were recorded using a SALS shadowgraph technique. We used both the static and dynamic structure factor algorithms for image processing in order to compute the structure factor and the correlation time of the fluctuations. The correlation time allowed us to estimate the diffusion coefficients of all three nanocolloids.
Out-of-equilibrium dynamics and extended textures of topological defects in spin ice
NASA Astrophysics Data System (ADS)
Udagawa, M.; Jaubert, L. D. C.; Castelnovo, C.; Moessner, R.
2016-09-01
Memory effects have been observed across a wide range of geometrically frustrated magnetic materials, possibly including Pr2Ir2O7 where a spontaneous Hall effect has been observed. Frustrated magnets are also famous for the emergence of topological defects. Here we explore how the interaction between these defects can be responsible for a rich diversity of out-of-equilibrium dynamics, dominated by topological bottlenecks and multiscale energy barriers. Our model is an extension of the spinice model on the pyrochlore lattice, where farther-neighbor spin interactions give rise to a nearest-neighbor coupling between topological defects. This coupling can be chosen to be "unnatural" or not, i.e., attractive or repulsive between defects carrying the same topological charge. After applying a field quench, our model supports, for example, long-lived magnetization plateaux, and allows for the metastability of a "fragmented" spin liquid, an unconventional phase of matter where long-range order co-exists with a spin liquid. Perhaps most strikingly, the attraction between same-sign charges produces clusters of these defects in equilibrium, whose stability is due to a combination of energy and topological barriers. These clusters may take the form of a "jellyfish" spin texture, centered on a hexagonal ring with branches of arbitrary length. The ring carries a clockwise or counterclockwise circular flow of magnetization. This emergent toroidal degrees of freedom provide a possibility for time-reversal symmetry breaking with potential relevance to the spontaneous Hall effect observed in Pr2Ir2O7 .
Thermal conductivity of glassy GeTe4 by first-principles molecular dynamics.
Bouzid, Assil; Zaoui, Hayat; Luca Palla, Pier; Ori, Guido; Boero, Mauro; Massobrio, Carlo; Cleri, Fabrizio; Lampin, Evelyne
2017-03-29
A transient thermal regime is achieved in glassy GeTe4 by first-principles molecular dynamics following the recently proposed "approach-to-equilibrium" methodology. The temporal and spatial evolution of the temperature do comply with the time-dependent solution of the heat equation. We demonstrate that the time scales required to create the hot and the cold parts of the system and observe the resulting approach to equilibrium are accessible to first-principles molecular dynamics. Such a strategy provides the thermal conductivity from the characteristic decay time. We rationalize in detail the impact on the thermal conductivity of the initial temperature difference, the equilibration duration, and the main simulation features.