A transferable intermolecular potential for nitramine crystals

We have investigated the transferability of a Buckingham repulsion-dispersion intermolecular potential previously developed [J. Phys. Chem. B 1997, 101, 798] for the explosive hexahydro-1,3,5-trinitro-1,3,5-s-triazine (RDX) to predict the crystal structures (within the approximation of rigid molecules) of 30 nitramines, comprising acyclic, monocyclic, and polycyclic molecules. It is shown that this potential model accurately reproduces the experimentally determined crystallographic structures and lattice energies of these crystals. For the majority of these crystals, the best agreement with experiment is obtained when the electrostatic charges are determined using ab initio methods that include electron correlation effects, namely, MP2 and B3LYP. The use of the electrostatic charges calculated at the Hartree-Fock level results in large deviations of the predicted lattice energies from the experimental values. These deviations of the lattice energies can be significantly decreased, without significantly affecting the predicted crystallographic parameters, by scaling the electrostatic charges with a constant factor.