title: Ground-state features in the THz spectra of molecular clusters of β-HMX.
abstract: We present calculations of absorption spectra arising from molecular vibrations at THz frequencies for molecular clusters of the explosive HMX using density functional theory (DFT). The features of these spectra can be shown to follow from the coupling of vibrational modes. In particular,  the coupling among ground-state vibrational modes provides a reasonable molecular-level interpretation of spectral features associated with the vibrational modes of molecular clusters. THz excitation from the ground state is associated with frequencies that characteristically perturb molecular  electronic states, in contrast to frequencies, which are usually substantially above the mid-infrared (mid-IR) range, that can induce appreciable electronic-state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations  about ground-state minima of molecules, either isolated or comprising a cluster, and THz absorption spectra. The DFT software program GAUSSIAN was used for the calculations of the absorption spectra presented here.
authors.affiliation: Naval Research Laboratory, Washington, D.C. 20375, USA.
authors.forename: Lulu
authors.initials: L
authors.lastname: Huang
authors.forename: Andrew
authors.initials: A
authors.lastname: Shabaev
authors.forename: Samuel G
authors.initials: SG
authors.lastname: Lambrakos
authors.forename: Lou
authors.initials: L
authors.lastname: Massa