PERIODIC HARTREE-FOCK CALCULATIONS ON CRYSTALLINE HCN

Periodic Hartree-Fock calculations on the tetragonal, high temperature , form of solid HCN are HF/6-31G* methodology, the lattice energy per molecule is predicted to be 21.8 kJ mol(-1), in good agreement with a n experimental value of 19 kJ mol(-1). Atoms-in-molecules decompositio n of the resulting charge distribution, based on Bader's zero-flux par titioning scheme, yields an in-crystal dipole moment of 1.734 a.u., a 38% increase over the isolated molecule. It is shown that this dipole moment enhancement is due solely to increased interatomic charge trans fer, since the atomic dipoles are actually reduced by the crystal fiel d. The intermolecular interactions in the crystal are characterized by means of the topology of the charge density; each chain is bound by r elatively strong N ... H hydrogen bonds, while the chains are linked b y weaker C ... N interactions. The effects of the crystal field on top ological properties of rho and del(2) rho are explored, and are found to cause changes of up to 10% compared to the gas phase. (C) 1996 Amer ican Institute of Physics.