Test of a simple and flexible S8 model molecule in alpha-s8 crystals

TL;DR

This study evaluates a simple molecular model of alpha-S8 crystals through MD simulations, confirming its accuracy at ambient conditions but revealing inaccuracies at low temperatures where it predicts non-existent phase transitions.

Contribution

It introduces and tests a simple, flexible S8 molecule model for alpha-S8 crystals, highlighting its strengths at standard conditions and limitations at low temperatures.

Findings

Model performs well at STP and T>~300 K

Fails at low temperatures, predicting false phase transition

Provides insights into molecular modeling of sulfur crystals

Abstract

Alpha S8 is the most stable crystalline form, at ambient pressure and temperature (STP), of elemental sulfur. In this paper we analyze the zero pressure low temperature part of the phase diagram of this crystal, in order to test a simple and flexible model molecule. The calculations consist in a series of molecular dynamics (MD) simulations, performed in the constant pressure- constant temperature ensemble. Our calculations show that this model, that gives good results for three crystalline phases at STP and T>~300 K, fails at low temperatures, predicting a structural phase transition at 200 K where there should be none.