title: Free-energy calculations using classical molecular simulation: application to the determination of the melting point and chemical potential of a flexible RDX model.
abstract: We present an extension of various free-energy methodologies to determine the chemical potential of the solid and liquid phases of a fully-flexible molecule using classical simulation. The methods are applied to the Smith-Bharadwaj atomistic potential representation of cyclotrimethylene trinitramine (RDX), a well-studied energetic material, to accurately determine the solid and liquid phase Gibbs free energies, and the melting point (Tm). We outline an efficient technique to find the absolute chemical potential and melting point of a fully-flexible molecule using one set of simulations to compute the solid absolute chemical potential and one set of simulations to compute the solid-liquid free energy difference. With this combination, only a handful of simulations are needed, whereby the absolute quantities of the chemical potentials are obtained, for use in other property calculations, such as the characterization of crystal polymorphs or the determination of the entropy. Using the LAMMPS molecular simulator, the Frenkel and Ladd and pseudo-supercritical path techniques are adapted to generate 3rd order fits of the solid and liquid chemical potentials. Results yield the thermodynamic melting point Tm = 488.75 K at 1.0 atm. We also validate these calculations and compare this melting point to one obtained from a typical superheated simulation technique.
authors.affiliation: U.S. Army Research Laboratory, Weapons and Materials Research Directorate, RDRL-WML-B, Aberdeen Proving Ground, MD, USA 21005. john.k.brennan.civ@mail.mil.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Sellers
authors.affiliation: Laboratory of Chemistry and Physics of Aerosols, Institute of Chemical Process Fundamentals of the ASCR, v. v. i., Rozvojová 135/1, 165 02 Prague 6-Suchdol, Czech Republic and Department of Physics, Faculty of Science, J. E. Purkinje University, České Mládeže 8, 400 96 Ústí n. Lab., Czech Republic.
authors.forename: Martin
authors.initials: M
authors.lastname: Lísal
authors.affiliation: U.S. Army Research Laboratory, Weapons and Materials Research Directorate, RDRL-WML-B, Aberdeen Proving Ground, MD, USA 21005. john.k.brennan.civ@mail.mil.
authors.forename: John K
authors.initials: JK
authors.lastname: Brennan