The PI will perform theoretical studies of the high pressure properties of candidate mantle and core phases in order to better constrain the chemistry and mineral composition of the mantle and core. Several important and basic questions will be addressed, including: 1) phase transitions in SiO2 stishovite that would lead to extreme shear elastic softening, 2) possible high temperature phase transitions in MgSiO3, 3) bonding characteristics of iron in search for new candidate high pressure-high temperature phases.