BREATHING SHELL-MODEL IN MOLECULAR-DYNAMICS SIMULATION - APPLICATION TO MGO AND CAO

Molecular dynamics (MD) simulation is used to calculate the elastic co nstants of both MgO and CaO at zero pressure, and their temperature de pendences, as well as the temperature-pressure-volume equation of stat es of the two oxides. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals, and repulsive interaction s. In order to account for the observed large departures from the Cauc hy relation of the elastic constants of the two oxides, the breathing shell model (BSM) is introduced in MD simulation, in which the repulsi ve radii of O ions are allowed to deform isotropically under the effec ts of other ions in the crystal, with each core and breathing shell be ing linked by a harmonic spring with force constant k. Required energy parameters, including k, were derived empirically to reproduce the ob served molar volumes and elastic constants of the two oxides at ambien t conditions, and their temperature dependences as accurately as possi ble. The MD simulation with BSM is very satisfactory in reproducing ac curately not only the observed large Cauchy violations,but also the me asured molar volumes and individual elastic constants of the two oxide s over wide temperature ranges, at zero pressure. In addition the MD s imulation reproduces accurately the measured volume compression data o f CaO up to 60 GPa at 300 K. Here we present the MD simulated temperat ure-pressure-volume equation of state of CaO as a useful internal pres sure calibration standard at high temperatures and high pressures. (C) 1998 American Institute of Physics.