Free energy calculations of elemental sulphur crystals via molecular dynamics simulations

TL;DR

This study compares two free energy calculation methods for sulfur crystals using molecular dynamics simulations, highlighting the accuracy and efficiency of a vibrational density of states-based estimation.

Contribution

It introduces and compares a fast vibrational mode-based free energy estimation with the traditional thermodynamic integration method for sulfur crystals.

Findings

The vibrational density of states method provides reliable free energy estimates.

Both methods yield consistent results for alpha- and alpha'- S8 crystals.

The fast estimation method is computationally efficient and practical.

Abstract

Free energy calculations of two crystalline phases of the molecular compound S8 were performed via molecular dynamics simulations of these crystals. The elemental sulphur S8 molecule model used in our MD calculations consists of a semi-flexible closed chain, with fixed bond lengths and intra-molecular potentials for its bending and torsional angles. The intermolecular potential is of the atom-atom Lennard-Jones type. Two free energy calculation methods were implemented: the accurate thermodynamic integration method proposed by Frenkel and Ladd and an estimation that takes into account the contribution of the zero point energy and the entropy of the crystalline vibrational modes to the free energy of the crystal. The last estimation has the enormous advantage of being easily obtained from a single MD simulation. Here we compare both free energy calculation methods and analyze the reliability of the fast estimation via the vibrational density of states obtained from constrained MD simulations. New results on alpha- and alpha'- S8 crystals are discussed