q-GRID: A New Method To Calculate Lattice and Interaction Energies for Molecular Crystals from Electron Densities

We present a new method to calculate lattice and intermolecular interaction energies for molecular crystals from electron densities obtained within the crystalline environment: q-GRID. The electron density is partitioned over a grid, and each grid point is assigned to a specific molecule. Intermolecular interaction energies are calculated as a sum of Coulomb interactions between grid points and nuclei of pairs of molecules and analytical dispersion and repulsion contributions. An advantage of this method is that the interactions within a molecule are automatically excluded. After a description of the new method and the computational setup, three test cases representing different classes of molecular crystals are presented: anthracene, isonicotinamide, and DL-methionine. For the polymorphic compounds, q-GRID is able to obtain the correct ranking of the polymorphic stability. Calculated lattice energies, as a sum of intermolecular interactions, are in good agreement with sublimation enthalpies. The code of q-GRID is made publicly available.