The vibrational, thermodynamic and mechanical properties of four types HMX based on the first-principles study

The vibrational, thermodynamic and mechanical properties of alpha-, beta-, delta-, and gamma-HMX are investigated based on the first principle method. The calculated lattice parameters of four types HMX based on the GGA-PBE+G functional are in good agreement with the experimental values and our pervious calculated values. The phonon density of states and vibrational properties of HMX are obtained. In the lower or higher vibrational region, the molecular motions are in certain NO2 or CH2 group. In the middle region, the motions are widely distributed in different group. Besides, the thermodynamic properties such as Helmholtz free energy (F), entropy (S), enthalpy (H), Gibbs free energy (G) and heat capacities (C-v) are obtained. The results of Debye temperatures show that the most stable beta-HMX is going to be unstable when the temperature increases. The elastic constants of alpha-, beta- and delta-HMX are calculated and compared with previous calculated results. The elastic constants of gamma-HMX are obtained for the first time. Furthermore, the mechanical properties including bulk modulus (B), shear modulus (S), Young's modulus (G), Poisson's ratio (nu), B/G value and universal anisotropy index (A(U)) are also calculated. We find that alpha-HMX and beta-HMX possess brittle behavior while delta-HMX and gamma-HMX are ductile. The anisotropy of gamma-HMX is the smallest and the anisotropy of beta-HMX is the biggest.