Thermodynamic properties of solid sodium from quasiharmonic lattice dynamics and molecular dynamics

Quasiharmonic-lattice-dynamics and molecular-dynamics calculations were performed on metallic Na from the low-temp. region to above melting at several different volumes. A pseudopotential model was used that consisted of a large volume--dependent potential plus a small effective two-body potential. From the molecular-dynamics results for the solid phase, the Helmholtz free energy was constructed and the thermodynamic properties up to the melting temp. were calculated. The anharmonic contributions to the internal energy and pressure are determined directly from molecular dynamics without using thermodynamic perturbation theory. Calculated and experimental values of the zero-pressure volume-temp. curve, isothermal bulk modulus, heat capacity and Gruneisen parameter are found to be in good agreement. The combination of quasiharmonic-lattice-dynamic theory in the quantum regime and molecular dynamics in the classical regime provide a simple and natural representation of the vibrational thermodynamics of a solid. 22 ref.--AA