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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unit cell parameter changes so, the frequencies of vibrational modes also change even in constant volume and the QHA is no longer valid.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

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