The composition of the lower mantle can be investigated by examining densities and seismic velocities of compositional models as functions of depth. In order to do this it is necessary to know the volumes and thermoelastic properties of the compositional constituents under lower mantle conditions. We determined the thermal equation of state (EOS) of MgSiO3 perovskite using the nonempirical variational induced breathing (VIB) interatomic potential with molecular dynamics simulations at pressures and temperatures of the lower mantle. We fit our pressure-volume-temperature results to a thermal EOS of the form P(V,T)=P0(V,T0)+ΔPth(T), where T0=300 K and P0 is the isothermal Universal EOS. The thermal pressure ΔPth can be represented by a linear relationship ΔPth=a+bT. We find V0=165.40 ¿3, KT0=273 GPa, KT0'=3.86, a=-1.99 GPa, and b=0.00664 GPa K-1 for pressures of 0--140 GPa and temperatures of 300--3000 K. By fixing V0 to the experimentally determined value of 162.49 ¿3 and calculating density and bulk sound velocity profiles along a lower mantle geotherm we find that the lower mantle cannot consist solely of (Mg, Fe)SiO3 perovskite with XMg ranging from 0.9--1.0. Using pyrolitic compositions of 67 mol % perovskite (XMg=0.93--0.96) and 33 mol % magnesiow¿stite (XMg=0.82--0.86), however, we obtained density and velocity profiles that are in excellent agreement with seismological models for a reasonable geotherm. ¿ 2001 American Geophysical Union