Molecular Packing and Molecular Dynamics Study of the Transferability of a Generalized Nitramine Intermolecular Potential to Non-Nitramine Crystals

Abstract

We have analyzed the transferability of a previously proposed intermolecular potential for nitramine crystals to reproduce the experimentally determined crystal structures (within the approximation of rigid molecules) of 51 nitro compounds. These compounds include different types of acyclic, monocyclic, and polycyclic molecules. It is shown that this potential model accurately reproduces the experimentally determined crystallographic structures and lattice energies for the majority of these crystals. The best agreement with experimental structural and energetic data is obtained when the electrostatic charges have been 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 differences between the predicted and the experimental values of the lattice energies. This difference can be significantly decreased by scaling the electrostatic charges with a general factor without introducing significant variations of the predicted crystallographic parameters. Further testing of the proposed intermolecular potential has been done by performing isothermal−isobaric molecular dynamics (MD) simulations over the temperature range 100−450 K, at atmospheric pressure, for the monoclinic phase of the 2,4,6-trinitrotoluene (TNT) crystal and for the polymorphic phase I of the pentaerythritol tetranitrate (PETN I) crystal. In each case, the results show that throughout the MD simulations the average structures of the crystals maintain the same space group symmetry as the one determined experimentally and there is a good agreement between the calculated crystallographic parameters and the experimental values. The thermal expansion coefficients calculated using the present model indicate an overall anisotropic behavior for both TNT and PETN I, with a thermal isotropy for PETN I along cell directions a and b.