Calculating the vibrational thermodynamic properties of bulk oxides using lattice dynamics and molecular dynamics

In this paper we outline an approach for calculating the vibrational thermodynamic properties of an inorganic solid from a molecular dynamics simulation and then compare them with those evaluated, using the more established lattice dynamics approach. Our motivations are twofold. First, lattice dynamics is impractical for simulations of more than a few hundred atoms, whereas molecular dynamics can readily be applied to systems of several thousand atoms. Second, lattice dynamics incorporates a quasiharmonic approximation and is therefore unreliable when anharmonic effects dominate. The vibrational properties of three oxides, MgO, TiO2, and Fe2O3, were calculated over a range of temperatures between 300 K and 1500 K. The results show good agreement in their predicted phonon density of states and in the calculated vibrational contribution to the free energy over the entire temperature range considered.