Comparative study of quasiharmonic lattice dynamics, molecular dynamics and Debye model applied to MgSiO3 perovskite

We consider three independent methodologies for calculating thermal equatio n of state (EOS) of the major earth-forming mineral, orthorhombic MgSiO3 pe rovskite: molecular dynamics (MD), lattice dynamics (LD) and Debye model (D M). Using the most recent developments in the GULP code, we derive a new in teratomic potential, which is demonstrated to be extremely robust at both h igh temperatures and high pressures. With this potential we construct a qua siharmonic self-consistent DM based on elastic properties of the crystal, a nd compare its results with results of more rigorous LD and MD simulations with the same potential model. We show that the DM reproduces quite accurat ely harmonic constant-volume heat capacity above 500 K, but gives thermal e xpansion and Gruneisen parameter (gamma) that are too small. We conclude th at MgSiO3 perovskite is not a Debye-like solid, in contrast to what has oft en been assumed in geophysical literature. Acoustic gamma, often used in ge ophysical studies, are a very elude approximation to the true gamma. To obt ain good accuracy, one needs to know the gamma (V) function more accurately than the DM can give. However, analytical functions, given by the Debye th eory, are useful for fitting thermal expansion and related parameters at el evated temperatures. A common assumption that q = d ln gamma /d In V is con stant is found to be inadequate: in fact, q varies strongly with volume and can reach negative values towards the base of the lower mantle. This can b e relevant for discussion of the anomalous properties of the core-mantle bo undary (D ") layer. Comparison of results of LD and MD indicates importance of intrinsic anharmonic contributions in the thermal expansion and gamma. Therefore, MD is the most suitable technique for simulating minerals at the Earth's mantle conditions. (C) 2000 Elsevier Science B.V. All rights reser ved.