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We present here molecular-dynamics simulation results of the vapor-liquid coexistence curve, surface tension, and self-diffusion coefficients of sulfur hexafluoride. Sulfur hexafluoride is modeled as a rigid molecule, following the model proposed by Pawley [Mol. Phys. 43, 1321 (1981)]. Vapor-liquid coexistence curve and surface tension are obtained through direct molecular-dynamic simulations in the NVT ensemble. Simulation results are able to reproduce the qualitative shape of the vapor-liquid envelope. However, lower densities, a higher critical temperature, and an overestimated surface tension are obtained here. Those deviations are explained on the basis of the rigidity of the molecular model used. Self-diffusion coefficients are calculated from simulations in the NVE ensemble for different gas states at atmospheric pressure. The rigid model performs better for dynamical properties since simulation results provide very good agreement with available experimental data in this case.
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PMID:Sulfur hexafluoride's liquid-vapor coexistence curve, interfacial properties, and diffusion coefficients as predicted by a simple rigid model. 1632 Nov 1

A modification of the constrained equations of motion of Kalibaeva et al. [Mol. Phys. 101, 765 (2003)] in the NPH and NPT ensembles is presented. The modified equations of motion are discretized using central-difference techniques, and the derived integrators are time reversible and conserve the invariant phase space measure. The constraint algorithm builds on the work of Toxvaerd et al. [J. Chem. Phys. 131, 064102 (2009)] in the NVE and NVT ensembles: it thus conserves the holonomic bond constraints at the finite machine precision level in the NPH and NPT ensembles. The algorithms were tested on a system of n=320 ortho-terphenyl molecules, arriving at the target temperature and pressure in a low and high pressure state. Isobaric heat capacities in the NPH and NPT ensembles were calculated for comparison using the fluctuation formulas as well as the thermodynamic definition. The heat capacities agree within the estimated uncertainties.
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PMID:Time reversible molecular dynamics algorithms with holonomic bond constraints in the NPH and NPT ensembles using molecular scaling. 2042 67

Molecular dynamics (MD) simulations were performed to study the migration of 2,4,6-trinitrotoluene (TNT) in the fluorine rubber binder of polymer-bonded explosives (PBX) over a wide range of temperatures. The diffusion coefficient (D) of TNT is determined via microcanonical (NVE) MD simulation using the COMPASS force field. The calculated diffusion coefficient (D) was then used to compute the migration time of TNT based on Fick's second law and the results agree well with the experimental data. The relation between D of TNT and temperature was confirmed and the results confirm the temperature-dependence of diffusion coefficients of TNT in the binder, but a break is seen about the melt point (the temperature at which the elastomeric state of the binder changes to a viscosity state) in the Arrhenius plot of ln(D) versus 1/T.
J Mol Model 2011 Nov
PMID:Theoretical study on the diffusive transport of 2,4,6-trinitrotoluene in polymer-bonded explosive. 2136 Jan 79

Direct NDDO-based Born-Oppenheimer molecular dynamics (MD) have been implemented in the semiempirical molecular orbital program EMPIRE. Fully quantum mechanical MD simulations on unprecedented time and length scales are possible, since the calculation of self-consistent wavefunctions and gradients is performed in a massively parallel manner. MD simulations can be performed in the NVE and NVT ensembles, using either deterministic (Berendsen) or stochastic (Langevin) thermostats. Furthermore, dynamics for condensed-phase systems can be performed under periodic boundary conditions. We show three exemplary applications: the dynamics of molecular reorganization upon ionization, long timescale dynamics of an endohedral fullerene, and calculation of the vibrational spectrum of a nanoparticle consisting of more than eight hundred atoms. Graphical AbstractA snapshot from an MNDO-H simulation of NH4+@C60 at 4000 K shortly before a proton crosses the fullerene wall to give NH3@C60H+.
J Mol Model 2020 Feb 03
PMID:EMPIRE: a highly parallel semiempirical molecular orbital program: 3: Born-Oppenheimer molecular dynamics. 3200 84