  Understanding the fundamental processes of light-matter interaction is
important for detection of explosives and other energetic materials, which are
active in the infrared and terahertz (THz) region. We report a comprehensive
study on electronic and vibrational lattice properties of structurally similar
1,3-dinitrobenzene (1,3- DNB) crystals through first-principles electronic
structure calculations and THz spectroscopy measurements on polycrystalline
samples. Starting from reported x-ray crystal structures, we use
density-functional theory (DFT) with periodic boundary conditions to optimize
the structures and perform linear response calculations of the vibrational
properties at zero phonon momentum. The theoretically identified normal modes
agree qualitatively with those obtained experimentally in a frequency range up
to 2.5 THz and quantitatively at much higher frequencies. The latter
frequencies are set by intra-molecular forces. Our results suggest that van der
Waals dispersion forces need to be included to improve the agreement between
theory and experiment in the THz region, which is dominated by intermolecular
modes and sensitive to details in the DFT calculation. An improved comparison
is needed to assess and distinguish between intra- and intermolecular
vibrational modes characteristic of energetic materials.
