DFT studies of pressure effects on structural and vibrational properties of crystalline octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine

摘要

A detailed study of the structural, electronic, and vibrational properties of crystalline octahydro1,3,5,7-tetranitro-l,3,5,7-tetrazocine (HMX) under hydrostatic pressure of 0-100 GPa was performed with density functional theory (DFT). The results show that the compressibility of HMX crystal is anisotropic. With the increasing pressure, the lattice constants and cell volumes calculated by local density approximation (LDA) gradually approach these by the PW91 functional of generalized gradient approximation (GGA). The band gap reduction is more pronounced in the low-pressure range compared to the high-pressure region. The band gaps calculated by LDA and GGA pseudopotential plane-wave reproduce the trend of pressure-induced variation of band gap by all electron calculations. The calculated pressureinduced frequency shifts indicate that the pressure produces a more significant influence on the ring deformation and stretching vibrations than on other modes. The vibrational modes associated with the motions of the CH2 and NO2 side groups are quite sensitive to pressure. The mixing between different vibrational modes becomes stronger under compression. Our results also show that DFT can well describe the inter-molecular interactions in HMX under high pressure.