P. Ballone and R. O. Jones

It is now well established that the DFT method can track the small changes in the energy with unit cell size that are needed to compute the stiffness matrix, from which one can obtain all the elastic moduli of the crystal and other information such as the velocity of sound. The method has also recently been applied to thereto-mechanical properties such as the coefficient of thermal expansion. Such calculations are computationally intensive because they involve the determination of derivatives of the phonon frequencies with respect to the volume. It is therefore useful to consider simpler methods that may give results of lower, but yet useful accuracy. One such method is assumes a Debye spectrum for the phonons and involves an averaged Gruneisen parameter. Such a method was proposed many years ago, but its consequences have not been explored to any great extent. The two essential elements that are missing are a a) a reliable, efficient computational scheme for the relevant parameters and b) a large number of computations over a wide variety of materials to establish when the method is reliable and when it is not.