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 $P=400$ 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.