  We report a detailed theoretical study of the structural, vibrational, and
optical properties of solid nitromethane using first principles density
functional calculations. The ground state properties were calculated using a
plane wave pseudopotential code with either the local density approximation
(LDA), the generalized gradient approximation (GGA), or with a correction to
include van derWaals interactions. Our calculated equilibrium lattice
parameters and volume using a dispersion correction are found to be in
reasonable agreement with the experimental results. Also, our calculations
reproduce the experimental trends in the structural properties at high
pressure. It was found to be a discontinuity in the bond length, bond angles
and also a weaking of hydrogen bond strength in the pressure range from 10 to
12 GPa, picturing the structural transition from phase I to Phase II. Moreover,
we predict the elastic constants of solid nitromethane and found that the
corresponding bulk modulus is in good agreement with experiments. The
calculated elastic constants are showing an order of C11> C22 > C33, indicating
that the material is more compressible along the c-axis. We also calculated the
zone center vibrational frequencies and discuss the internal and external modes
of this material under pressure. From this, we found the softing of lattice
modes around 8 to 12 GPa. We have also attempt the quasiparticle band structure
of solid nitromethane with the G0W0 approximation and found that nitromethane
is an indirect band gap insulator with a value of the band gap of about 7.8 eV
with G0W0 approximation. Finally, the optical properties of this material,
namely the absorptive and dispersive part of the dielectric function, and the
refractive index and absorption spectra are calculated and the contribution of
different transition peaks of the absorption spectra are analyzed.
