Ab initio determination of the melting point of aluminum by thermodynamic integration

The melting point of aluminum has been obtained in an ab initio molecular dynamics calculation by determination of the free energies of the solid and liquid phases as a function of temperature along the zero pressure isobar. The focus of the article is to demonstrate the problems which can arise in obtaining adequately sampled free energies. The time scale on which "adiabatic switching" may be performed to calculate the free energy of the ab initio system relative to a classical reference state is discussed. To provide a consistency check, two reference states for this thermodynamic integration are used, the one component plasma and the Lennard-Jones (LJ) system. These illustrate particular difficulties which can arise. In the LJ case, for example, the intermediate fluid states which arise in integrating from the LJ fluid to the full ab initio description of Al are found to freeze. Ultimately, consistent results are obtained. (C) 2000 American Institute of Physics. [S0021- 9606(00)70438-7].