Dispersion Interactions in Calculations of Properties of Energetic Materials

Until recently, first-principles calculations of potential energy surfaces (PES) were restricted to intermolecular interactions involving molecules containing just several atoms. This was due on one hand to high costs of wave-function-based electronic structure methods and, on the other hand, to the failure of the density functional theory (DFT) approaches to reproduce the dispersion part of intermolecular interactions. One solution to this problem is symmetry-adapted perturbation theory based on DFT description of monomers [SAPT(DFT)]. In applications to energetic materials, SAPT(DFT) predicted the correct crystal structure of RDX (1,3,5-trinitroperhydro-1,3,5-triazine). Recently, the complete PES of FOX-7 (1,1-diamino-2,2-dinitroethene) dimer was obtained using SAPT(DFT). Preliminary molecular dynamics simulations of the FOX-7 crystal show an improved agreement with experiment compared to literature results. A recently developed nearly-linearly scaling implementation of the SAPT(DFT) dispersion energy has been applied to interactions of energetic molecules. When the development of linearly-scaling SAPT(DFT) is finished, accurate studies of energetic molecules significantly larger than RDX and of other important systems (including biomolecules), containing in excess of one hundred atoms, will be possible. Another approach which can be applied to such systems is the dispersionless density functional (dlDF) method developed in our group which reproduces interaction energies with the dispersion component removed. The dispersion energy is then computed from an asymptotic function fitted to SAPT(DFT) dispersion energies of a training set, resulting in a method denoted as dlDF+D. Cross sections of the PES of the HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane) dimer calculated using dlDF+D are presented.