First-Principles Lattice Energy Calculation of Urea and Hexamine Crystals by a Combination of Periodic DFT and MP2 Two-Body Interaction Energy Calculations

Abstract

This article proposes a new method for the accurate evaluation of the lattice energies (stabilization energies associated with the formation of crystals from isolated molecules) of molecular crystals by a combination of DFT and MP2 calculations. The interactions of well-separated molecules were evaluated by periodic DFT calculations. The interactions with neighboring molecules were evaluated by MP2-level two-body interaction energy calculations with the neighboring molecules. The sublimation energies calculated for crystals of urea and hexamine using the proposed method (21.2 and 20.0 kcal/mol, respectively) were close to the experimental values (20.9-23.6 and 17.7-18.8 kcal/mol, respectively). The lattice energies calculated for the two crystals by the proposed method are significantly different from those obtained by DFT calculations, suggesting that the dispersion contribution to the lattice energy is significant even in the crystal, where molecules are bound by hydrogen bonds. Lattice energies calculated by changing the range of the neighboring molecules show that the nearest-neighboring molecules are mainly responsible for the dispersion interactions in the crystals.