The effect of Coulombic interactions on the calculated crystal structures of benzene at atmospheric and 25 kbar pressure

The crystal structures of benzene I (orthorhombic) and high-pressure benzene II (monoclinic) were calculated at atmospheric pressure and 25 kbar pressure by minimization of the intermolecular lattice energy subject to the pressure constraint. The first set of calculations used an atom-atom (exp-6) potential with no net coulombic charges on the atoms. The second set of calculations used an (exp-6-1) potential which placed charges of ± 0.18 electron on each atom of the benzene molecule. The no-charge calculation incorrectly predicted that benzene should be monoclinic at both atmospheric and 25 kbar pressure. The coulombic charge calculation correctly predicted the observed crystal transaction, favoring the orthorhombic structure at atmospheric pressure but favoring the monoclinic structure at 25 kbar. In the latter calculations there were substantial coulombic contributions to the lattice energy.