text: The thermal pressure of a crystal defines how the unit-cell parameters change as a function of pressure and temperature. Therefore, it also controls how the cell parameters change along an isochore, namely as a function of . Usually, Mie-Grüneisen-Debye and other Quasi harmonic approximation (QHA) based state functions are being used to estimate volumes and densities of mineral phases in diverse applications such as thermodynamic, deep-Earth geophysical models and other planetary bodies. In the case of isotropic (or approximately isotropic) thermal pressure, the unit cell parameter remains constant along the isochore and the QHA is valid. But when the thermal pressure is anisotropic, the unit cell parameter changes so, the frequencies of vibrational modes also change even in constant volume and the QHA is no longer valid.
  The combined effect of a change in pressure and temperature is described by the strain tensor :
  Where  is the volume thermal expansion tensor and  is the compressibility tensor. The line in the P-T space which indicates that the strain  is constant in a particular direction within the crystal is defined as:
  Which is an equivalent definition of the isotropic degree of thermal pressure.