COMPARISON BETWEEN THE LATTICE-DYNAMICS AND MOLECULAR-DYNAMICS METHODS - CALCULATION RESULTS FOR MGSIO3 PEROVSKITE

The lattice dynamics (LD) and molecular dynamics (MD) methods have been used to calculate the structure, bulk modulus, and volume thermal expansivity of MgSiO3 perovskite, in order to investigate the reliability of the two simulation techniques over a wide range of temperature and pressure conditions. At an intermediate temperature of 500 K and zero pressure, the LD and MD values are in exellent agreement for both the structure and bulk modulUS of MgSiO3 perovskite. At high temperatures and zero pressure, however, the LD method, which is based on the quasi-harmonic approximation, increasingly overestimates the molar volume of MgSiO3 perovskite because of the neglect of higher-order anharmonic terms. At the high temperatures and high pressures prevailing in the lower mantle, the errors in the LD values for both the molar volume and bulk modulus, relative to the MD values, are generally small or negligible. However, since anharmonicity decreases substantially with pressure but increases rapidly with temperature, the error in the LD simulated volume thermal expansivity is serious, especially in the lower pressure region.