Dispersion Interactions in Calculations of Properties of Energetic Materials
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD
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Until recently, first-principles calculations of potential energy surfaces PES were restricted to intermolecular interactions involving molecules containing just several atoms. On one hand, this was due to the high cost 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 SAPTDFT. In applications to energetic materials, SAPTDFT 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 SAPTDFT. Preliminary molecular dynamics simulations of the FOX-7 crystal show an improved agreement with experiment compared to literature results. A recently developed nearly-linear scaling implementation of the SAPTDFT dispersion energy has been applied to interactions of energetic molecules. When the development of linear-scaling SAPTDFT 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 dispersion-less 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 SAPTDFT dispersion energies of a training set, resulting in a method denoted as dlDFD. Cross sections of the PES of the HMX octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocane dimer calculated using dlDFD are presented.
- Physical Chemistry