The multi-coefficient correlated quantum mechanical calculations for structures, energies, and harmonic frequencies of HF and H2O dimers

The accurate determination of interaction energies and structures of hydrog en-bonded complexes has been an important issue of ab initio theory for a l ong time. Extensive theoretical studies have been performed to correct elec tronic correlation and the basis set truncation error (BSTE) that is a cons equence of the incompleteness of the one-electron basis set. We have used r ecently developed multilevel methods to calculate the structures, harmonic frequencies and the dissociation energies of the HF and water dimers. The s even multilevel methods, namely SAC-MP2/cc-pVDZ, SAC-MP4SDQ/cc-pVDZ, MC-QCI SD, MCCM-CO-MP2, MCCM-UT-MP4SDQ, MCCM-UT-CCSD, and MCG3, have been tested. The MC-QCISD, MCCM-UT-MP4SDQ, MCCM-UT-CCSD, and MCG3 method predict the str uctures and harmonic frequencies of HF and H2O dimers reasonably well compa red with experiments and high level ab initio results. Particularly, the MC CM-UT-MP4SDQ and MCCM-UT-CCSD methods show very good agreement of both the interfragment distances and the dissociation energies with experiments usin g significantly less cost than the high-level ab initio calculations. (C) 2 001 American Institute of Physics.