Direct Calculation of Configurational Entropy: Pair Correlation Functions and Disorder

Techniques such as classical molecular dynamics [MD] simulation provide ready access to the thermodynamic data of model material systems. However, the calculation of the Helmholtz and Gibbs free energies remains a difficult task due to the tedious nature of extracting accurate values of the excess entropy from MD simulation data. Thermodynamic integration, a common technique for the calculation of entropy requires numerous simulations across a range of temperatures. Alternative approaches to the direct calculation of entropy based on functionals of pair correlation functions [PCF] have been developed over the years. This work builds upon the functional approach tradition by extending the recently developed entropy pair functional theory [EPFT] to three new material systems. Direct calculations of entropy for the BCC iron and FCC copper (modeled with the modified embedded atom method [MEAM] potential) and the Diamond Cubic silicon system (modeled with the Tersoff potential) are compared against a target entropy as determined by thermodynamic integration. The sources of and correction to the high temperature error in several proposed functional approaches is explored in depth. Finally, a working code is provided to the community via Github to implement the extended EFPT to compute entropy using trajectory files generated from a single simulation.