THE PREDICTION OF MOLECULAR EQUILIBRIUM STRUCTURES BY THE STANDARD ELECTRONIC WAVE-FUNCTIONS

A systematic investigation has been carried out of the accuracy of mol ecular equilibrium structures of 19 small closed-shell molecules conta ining first-row atoms as predicted by the following standard electroni c nb initio models: Hartree-Fock (HF) theory, Moller-Plesset theory to second, third, and fourth orders (MP2, MP3, and MP4), coupled-cluster singles and doubles (CCSD) theory; CCSD theory with perturbational tr iples corrections [CCSD(T)], and the configuration-interaction singles and doubles (CISD) model. For all models, calculations were carried o ut using the correlation-consistent polarized valence double-zeta (cc- pVDZ) basis, the correlation-consistent polarized valence triple-zeta (cc-pVTZ) basis, and the correlation-consistent polarized valence quad ruple-zeta (cc-pVQZ) basis. Improvements in the basis sets shorten the bond distances at all levels. Going from cc-pVDZ to cc-pVTZ, bond dis tances are on the average reduced by 0.8 pm at the Hartree-Fock level and by 1.6 pm at the correlated levels. From cc-pVTZ to cc-pVQZ, the c ontractions are about ten times smaller and the cc-pVTZ basis set appe ars to yield bond distances close to the basis-set limit for all model s. The models HF, MP2, and CCSD(T) give improved accuracy at increased computational cost. The accuracy of the Moller-Plesset series oscilla tes, with MP3 being considerably less accurate than MP2 and MP4. The M P2 geometries are remarkably accurate, being only very slightly improv ed upon at the MP4 level for the cc-pVQZ basis. The CCSD equilibrium s tructures are only moderately accurate, being intermediate between MP2 and MP3. The accuracy of the CCSD(T) model, in contrast, is high and comparable to that observed in most experimental studies and it has be en used to challenge the experimentally determined equilibrium structu re of HNO. The CISD wave function provides structures of low quality. (C) 1997 American Institute of Physics.