Importance of Dispersion Forces for the Simulation of Amorphous Phase Change Materials

GeTe and GST amorphous materials are re-investigated using ab initio dft molecular dynamics to compare the effect of various functionals that include the treatment of dispersion (Van der Waals) forces on the structural and dynamical properties of the final amorphous structures. We show that the proportion of tetrahedral Ge to other types of environments as well as the ratio of 3-fold and 2-fold bonded Te atoms is much dependent on the choice of functional. The different functionals yield variable agreement with the available structural experimental data. Properties such as the diffusion coefficient and vibrational densities of states are computed, indicating that models that are extremely close in energy may have very different experimental signatures