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 super-position error whereas basis set optimization improved the quality Of the calculation relatively little. (C) 2004 Elsevier B.V. All rights reserved.