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A method for first principles predictions of observed temperature-dependentNQR spectra is presented using density functional theory (DFT) and the isobaricT-dependent NQR frequencies of 35Cl and 14N nuclei are computed for the twomolecular crystals (1) alpha-paradichlorobenzene, and (2) the nitroamine highexplosive beta-HMX (beta-octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) overa range of 200 K and up to room temperature. Notably, the method requires nosupposition of a form for the intermolecular potentials or wave functions,requires no particular insight as to the nature of the internal motions or ofthe chemical bonds present, and does not depend on the crystal structure,making the method amenable to any periodic solid for which experimentallydetermined structural data are available. For each substance, unit cells ofvarious volume are prepared using experimentally determined atomic positionsand cell parameters. In each of the prepared volume-corrected cells, amolecular dynamics (MD) simulation generates a set of perturbed atomicpositions, the collection of which is intended to represent the system at agiven T,V. For each configuration of atoms generated along the MD trajectory,the electric field gradient (EFG) tensors are computed at the site of eachquadrupolar nucleus. The rotational displacements of the moving EFG principalaxes from their equilibrium directions are used to apply a dynamic correctionto the DFT-computed static-lattice NQR frequencies, resulting infirst-principles DFT predictions of T-dependent NQR spectra at constantpressure. Because the V-dependence from thermal expansion and the T-dependencedue to internal motions are simultaneously considered, the NQR's notoriouslymodel-dependent temperature coefficients are computed entirely ab-initio.

DFT Calculations of Temperature-Dependent NQR Parameters in {\alpha}-paradichlorobenzene and {\beta}-HMX