THz Spectroscopy of 2,4,6-trinitrotoluene Molecular Solids from First Principles

We present a computational analysis of the THz spectrum of the monoclinic and orthorhombic polymorphs of 2,4,6-trinitrotoluene. Very good agreement with experimental data is found when using density functional theory that includes Tkatchenko-Scheffler pair-wise dispersion interactions. Furthermore, we show that for these polymorphs the theoretical results are only weakly affected by many-body dispersion contributions. Absence of dispersion interactions, however, causes sizable shifts in vibrational frequencies and directly affects the spatial character of the vibrational modes. Mode assignment allows a distinction between contributions of the monoclinic and orthorhombic polymorphs and shows that modes in the range of 0-3.3 THz are comprised of both inter- and intra-molecular vibrations, with the former dominating below 1.5 THz. Intra-molecular contributions primarily involve the nitro group. Finally, we present a prediction for the THz spectrum of 1,3,5-trinitrobenzene, showing that a modest chemical change leads to a markedly different THz spectrum.