Crystal Structures from Nonempirical Force Fields

Until recently, structure and properties of molecular crystals could not be predicted computationally since the forces acting between the constituent molecules in crystals were not known sufficiently accurately. This situation has changed with the development of an electronic structure method called symmetry-adapted perturbation theory based on the density-functional description of monomers [SAPT(DFT)]. This method is sufficiently efficient to be applied to interactions of energetic molecules, for example to cyclotrimethylene trinitramine (RDX). Systems even larger than RDX can be treated, for example, interaction energies for the dimer of perylene, containing 64 atoms, have been computed using SAPT(DFT). The SAPT(DFT) potential developed for RDX was used in a combined molecular packing, lattice energy minimization, and molecular dynamics approach to characterize low-energy polymorphs of the RDX crystal. The lowest-energy structure corresponded to the observed crystal and the results obtained for high-density polymorphs provide new information on the polymorphism of RDX. The SAPT(DFT) method should find important applications in development of new energetic materials, including crystal design, screening molecules for co-crystallization, and identification of low-energy polymorphs.