Phonon Anharmonicity In The Vibrational Entropy Of Transition Metals*

Evaluation of the Grüneisen parameter of transition metals employing the quasi-harmonic approximation (QHA) within density functional theory (DFT), yields systematically high values when compared to experimental data or with more direct methods like quantum-molecular dynamics (QMD) simulations. Using Tantalum as a test model for which a lot of experimental data exist, thermal properties were evaluated employing direct molecular dynamics (MD) simulations and compared to QHA predictions. The free energy and entropies are computed as a function of temperature and pressure employing the adiabatic switching formalism within classical MD. When compared with the quasi-harmonic entropy computed within QHA, the anharmonic entropy is large even at moderately low temperatures, suggesting that phonon frequencies are temperature dependent. The temperature contribution to the vibrational Grüneisen is significantly less important at high temperatures, where the volume-dependence contributions dominate.