A nice summary (in pdf format) by Michael Francis at Accelrys about the use of solid-state density functional theory (DFT) in the assignment of the HMX THz spectrum published in J. Phys. Chem. A earlier this year. Also, I'm on record again extolling the virtues of DMol3, having now used it in a number of inelastic neutron scattering studies, this crazy terahertz business, and for (an upcoming) examining the crystal environment of small polyoxometalates (which I wouldn't have even considered as early as two years ago given the state of computers way back when). Three more papers on the subject have come out since this case study was presented, all of which are linked and available in the terahertz spectroscopy category of this site.