An ab Initio Study of Solid Nitromethane, HMX, RDX, and CL20:  Successes and Failures of DFT

Using the PW91 and PBE density functional theories (DFT), we have studied four energetic molecular crystals, nitromethane, HMX, RDX, and CL20, with a wide range of basis sets. Intramolecular distances, simple angles, and band gaps are converged at plane wave cutoff energies (Ecut) of 430 to 495 eV. Cell parameters were determined over a range of Ecut values from 280 eV to 700 or 800 eV, depending upon the system. Lattice vectors, however, display large errors in the range of 0.2 Å to 1.0 Å (up to a 9.6% error at the highest Ecut used here), and a very slow convergence on basis set size. We hypothesize that the error in the lattice vectors is due to a lack of van der Waals forces in current DFT functionals. This deficiency will have unforeseen consequences on all crystal calculations for organic molecules, and therefore caution should be employed whenever interpreting results obtained from the current DFT functionals available in solid state codes. To properly describe the electronic structure of these types of crystals, these results suggest the need for new methods involving DFT to be developed which include accurate dispersion forces.