(262z) Accurate and Efficient Thermodynamic Properties from Molecular Simulation By Combining Lammps and Gomc

Many common applications of force fields involve molecular dynamics, but force field development is generally pursued within a Monte Carlo (MC) framework. This distinction can lead to dichotomous mindsets relating to subtle but significant details. For example, most force fields have been developed based on the assumption of spherical symmetry and tail corrections to compute the thermodynamic properties, which is quite convenient for MC simulation. Unfortunately, a fundamental issue arises in the context of molecular dynamics because the discontinuity in the force field undermines the conservation of energy, leading to a range of possible compensating treatments, and poor consistency from one implementation to the next. Furthermore, many common applications involve inhomogeneous fluids where this assumption breaks down. Although Ewald methods have been suggested as a remedy, our experience with these methods has left the issue unresolved.

In this work, we suggest the development of a Transferable Shifted Force Field based on the United Atom perspective (TranSFF-UA), taking the TraPPE-UA model as a starting point. We universally apply a cut and shift of 3Ï?, where Ï? is the position where the potential initially crosses zero. This seemingly simple change leads to remarkable difficulty when implemented in the commonly available simulation packages that we have considered: LAMMPS, GOMC, HOOMD-BLUE, and Cassandra.

This presentation focuses particularly on the promise and pitfalls of these prospective platforms. At first glance, LAMMPS offers the prospect of greatest consistency by developing the force field entirely within the molecular dynamics framework in which it is to be applied. This necessitates thermodynamic integration, however, which is quite feasible except where one might least suspect a problem: low density and high temperature. LAMMPS shows its greatest strength at low temperature and high density. On adapting for a shifted force, Cassandra and HOOMD-BLUE exhibit inconsistencies that remain unresolved after months (going on years) of interacting with these package developers. GOMC, on the other hand, is easily adapted for the shifted force while providing consistency with LAMMPS and resolving the issue of accuracy at high temperature and low density. At reduced temperatures greater than T/T_c = 0.7, the GCMC option of GOMC provides highly efficient generation of phase envelopes and saturation properties. To demonstrate this methodology, vapor pressure, saturated liquid density, and compressed fluid density are analyzed for n-alkanes, branched alkanes, ethers, olefins, naphthenics and aromatic compounds, and water.

This work is supported in part by the Scientific and Technological Research Council of Turkey (TUBITAK) through the grant no. 114M178.