Periodic Hartree–Fock calculations on crystalline HCN

Abstract

Periodic Hartree–Fock calculations on the tetragonal, high temperature, form of solid HCN are presented. Using HF/6-31G** methodology, the lattice energy per molecule is predicted to be 21.8 kJ mol−1, in good agreement with an experimental value of 19 kJ mol−1. Atoms-in-molecules decomposition of the resulting charge distribution, based on Bader’s zero-flux partitioning 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 transfer, since the atomic dipoles are actually reduced by the crystal field. The intermolecular interactions in the crystal are characterized by means of the topology of the charge density; each chain is bound by relatively strong N⋅⋅⋅H hydrogen bonds, while the chains are linked by weaker C⋅⋅⋅N interactions. The effects of the crystal field on topological properties of ρ and ∇2ρ are explored, and are found to cause changes of up to 10% compared to the gas phase.