A comparison of Hartree-Fock, MP2, and DFT results for the HCN dimer and crystal

A number of hydrogen-bond related quantities-geometries, interaction energies, dipole moments, dipole moment derivatives, and harmonic

vibrational frequencies-were calculated at the Hartree-Fock, MP2, and

different DFT levels for the HCN dimer and the periodic HCN crystal.

The crystal calculations were performed with the Hartree-Fock program

CRYSTAL92, which routinely allows an a posteriori electron-correlation

correction of the Hartree-Fock obtained lattice energy using different

correlation-only functionals. Here, we have gone beyond this procedure

by also calculating the electron-correlation energy correction during

the structure optimization, i.e., after each CRYSTAL92 Hartree-Fock

energy evaluation, the a posteriori density functional scheme was

applied. In a similar manner, we optimized the crystal structure at the

MP2 level, i.e., for each Hartree-Fock CRYSTAL92 energy evaluation, an

MP2 correction was performed by summing the MP2 pair contributions from

all HCN molecules within a specified cutoff distance. The crystal cell

parameters are best reproduced at the Hartree-Fock and the

nongradient-corrected HF + LDA and HF + VWN levels. The BSSE-corrected

MP2 method and the HF + P91, HF + LDA, and HF + VWN methods give

lattice energies in close agreement with the ZPE-corrected experimental

lattice energy. The (HCN), dimer properties are best reproduced at the

MP2 level, at the gradient-corrected DFT levels, and with the B3LYP and

BHHLYP methods. (C) 1996 John Wiley & Sons, Inc.