Abstract The parameter δ t is a dimensionless thermoelastic parameter important in thermodynamic studies involving high temperature at high pressure. We give a brief history of attempts to define δ T in terms of fundamental interatomic potentials. We show that ab initio calculations provide a way to find δ T(η, T) (where η = V V 0 ), since it is possible to find the minimum in the isochoric B T vs T data arising from the ab initio analysis ( B T is the isothermal bulk modulus). The method is demonstrated for MgO, where δ T is calculated over a wide η, T field. We find δ T decreases at high compression, but is independent of T. Knowledge of δ T(η, T) is important to find the high P-high T behavior of a number of important properties in thermal physics, including entropy, thermal expansivity, thermal pressure and the Grüneisen parameter.