In this new manuscript Ross Angel shows that for thermal-pressure Equations of State (EoS) such as the Mie-Grüneisen-Debye (MGD) the assumptions, including the quasi-harmonic approximation (QHA) and a simplified phonon density of states need to be carefully accounted for. In the manuscript is shown how the QHA is violated by materials exhibiting anisotropic thermal pressure. Is also shown that at pressures lower than those of the isochor of the reference volume, the static pressure may become sufficiently negative to make the compressional part of the EoS invalid.

(This article belongs to the Special Issue Mineral Physics—In Memory of Orson Anderson)