DFT Calculations of Motional Effects on NQR Parameters in Molecular Crystals*

The ³⁵Cl and ¹⁴N NQR frequencies in paradichlorobenzene and β- HMX are predicted at a variety of temperatures using density functional theory (DFT) and a general method is presented for first principles DFT calculations of the temperature dependent NQR spectra in molecular crystals. Crystal cells of various volumes corresponding to experimental cell parameters are derived and a sequence of atomic configurations are generated from molecular dynamics (MD) at each volume. The principal axes of the electric field gradient (EFG) tensors are computed for each configuration. The temperature dependence of the NQR frequencies are then computed geometrically from expressions relating the EFG tensor of a rotated system to that of a fixed system through rotations about the principle axes. The method simultaneously accounts for the volume dependence of the EFG due to thermal expansion, as well as the T-dependence of the EFG through internal motions, allowing a first-principles calculation of the isobaric NQR T-dependence using only experimental structural data. The method does not depend on the particulars of the crystal structure or require any insight as to the nature of the internal motions.