  The effect of pressure on the structural and vibrational properties of the
layered molecular crystal 1,1-diamino-2,2-dinitroethelene (FOX-7) are explored
by first principles calculations. We observe significant changes in the
calculated structural properties with different corrections for treating van
der Waals interactions to Density Functional Theory (DFT), as compared with
standard DFT functionals. In particular, the calculated ground state lattice
parameters, volume and bulk modulus obtained with Grimme's scheme are found to
agree well with experiments. The calculated vibrational frequencies
demonstrates the dependence of the intra and inter-molecular interactions in
FOX-7 under pressure. In addition, we also found a significant increment in the
N-H...O hydrogen bond strength under compression. This is explained by the
change in bond lengths between nitrogen, hydrogen and oxygen atoms, as well as
calculated IR spectra under pressure. Finally, the computed band gap is about
2.3 eV with GGA, and is enhanced to 5.1 eV with the GW approximation, which
reveals the importance of performing quasiparticle calculations in high energy
density materials.
