Within the harmonic approximation, the vibrational DOS is temperature independent, and any temperature depen-dence arises only through considering phonon occupation numbers. The quasi-harmonic approximation (QHA) goes beyond this harmonic model, by considering how phonon frequencies vary in response to thermal changes in equilibrium volume. Within the QHA, phonon frequencies at a given volume are treated within the harmonic approximation. The equilibrium volume at a given temperature can then be found by minimising Equation 2 with respect to volume at each temperature. The mode Grüneisen parameter, γ qλ provides a measure of anharmonicity with respect to changes in volume, and can be obtained from the gradient ∂ω qλ /∂V via