Development of computational methodologies for the prediction and analysis of solid-state terahertz spectra

The analytical applications of terahertz (THz) spectroscopy for the characterization of molecular solids have been limited by the lack of information concerning the assignment of observed spectral features to specific internal (intramolecular) and external (intermolecular) atomic motions. Computational methodologies addressing the assignment of spectral data are the enabling technology for moving THz spectroscopy to the forefront of available detection methods for both imaging and spectroscopic applications. Solid-state density functional theory (DFT) studies have been performed on the high explosives cyclotetramethylenetetranitramine (HMX) and pentaerythritol tetranitrate (PETN) in order to address the dependencies of the predictions of solid-state vibrations in the terahertz (3 to 120 cm^-1) region on the choice of basis set and integration grid size, building on previous work that examined this dependency on the choice of density functional. DFT THz simulations reveal that both the choice of basis set and grid size have important influences on the reproduction of spectral features. The sensitivity to basis set choice is most pronounced in the calculation of vibrational intensities, where it is found that THz absorption intensities are most accurately reproduced when derived from basis set-sensitive Mulliken atomic charges as opposed to basis set-insensitive atomic charges generated by the Hirshfeld partitioning method.