Ab initiocalculation of melting and thermodynamic properties of crystal and liquid aluminum

Abstract

A first-principles technique for calculating accurate equations of state of metals is illustrated with an application to Al. Results for the thermodynamic properties of Al at densities near normal, and at temperatures into the liquid phase, are presented. In contrast to previous calculations by other workers, the present technique makes no use of experimental data. The ab initio procedure begins with electronic-band-structure calculation of the ground-state energy, the elastic constants, and selected short-wavelength phonons. A physically realistic interatomic potential is calibrated by fitting it to the calculated elastic constants and phonons. Ion-motional contributions are computed from quasiharmonic lattice dynamics at low temperatures, and from molecular-dynamics simulations at classical temperatures. Melting is computed from the free energy constructed from these combined results. Very accurate results are obtained for zero-temperature properties and phonon frequencies. The calculated melting temperature of 955 K is within 2.5% of the observed value, a difference comparable to the estimated precision of the calculation. Good results are obtained for two highly sensitive quantities, the entropy change upon melting and the Clapeyron slope. The calculated entropy as a function of temperature is in excellent agreement with experiment for solid and liquid phases. The present procedure can potentially provide reliable values of thermodynamic properties of any metal under extreme conditions where no data are avilable.