Phase diagram of MgO from density-functional theory and molecular-dynamics simulations

We use first-principles methods ~no empirical parameters! to establish the phase diagram for the B1~NaCl!, B2~CsCl!, and liquid phases of MgO. We used density-functional theory with the generalized gradient approximation to predict the equation-of-state @volume versus pressure „V(P)...# at 0 K for MgO in the low-density B1 ~NaCl! phase and the high-density B2 ~CsCl! phase. We find a pressure-induced phase transition at P5400 GPa. We then fitted an MS-Q type force field ~FF! to the quantum results. This FF, denoted as qMS-Q FF, was then used in molecular dynamics ~MD! simulations to investigate the phase coexistence curves of the B1-B2 and B1-liquid phases. This leads to a first-principles phase diagram for MgO for pressures up to 500 GPa and temperatures up to 8000 K. The accuracy of the fit of the qMS-Q FF to the quantum mechanics validates the functional form of the qMS-Q FF in which the charges are obtained from charge equilibration ~QEq! and the nonelectrostatic forces are described with simple two-body Morse potentials. Such qMS-Q FF using no empirical data should be useful for MD or Monte Carlo simulations of many other materials. @S0163-1829~99!02346-2#