Anharmonicity in Equation of State Calculations: A Cross-Validation Study

The ability to accurately capture anharmonic effects is especially important for predicting and characterizing the behavior of materials at extreme conditions. Firstly, anharmonic contributions to the free energy can be both quantitatively and qualitatively significant for low Z materials and most materials at high enough temperatures. Secondly, since vibrational spectroscopy is one of the primary diagnostics for materials at extreme conditions, a proper inclusion of anharmonicity can sometimes be necessary to properly compare theoretical structure predictions to observed spectroscopic data. In this work, we use path integral molecular dynamics around the BCC-FCC transition in elemental lithium to benchmark and validate some of the main methods used in equation of state calculations: the vibrational self consistent field method, the 2PT model, and one variant of perturbation theory.