First-principles intermolecular binding energies in organic molecular crystals

The intermolecular binding (lattice) energies are calculated for the molecular crystals cyclotrimethylene trinitramine, pentaerythritol, and pentaerythritol tetranitrate using the Crystal 98/03 and Gaussian 98 programs, the Dmol program and the Castep program and compared with experiment. Calculating the theoretical intermolecular binding energy as a tool for testing the intrinsic quality of a calculation is explained. Optimization of the Gaussian basis sets for the Crystal 98/03 programs versus adding more terms to the Gaussian basis set is examined. Increasing the size of the basis set consistently reduces basis-set superposition error whereas basis set optimization improved the quality of the calculation relatively little.