``Comparison Between Molecular Geometry and Harmonic Vibrational Frequency Predictions from CISD[TQ] and CISDTQ Wavefunctions for Hydrogen Sulfide,'' B. C. Hoffman, C. D. Sherrill, and H. F. Schaefer, J. Chem. Phys. 107, 10616-10619 (1997).

The selection of which configurations to include in a configuration interaction (CI) wavefunction is a compromise between accuracy and computational difficulty. A compact and accurate configuration interaction wavefunction can be constructed by inclusion of all single and double excitations and certain triple and quadruple excitations chosen in an a priori manner according to how many electrons are placed in several subsets of orbitals. Such a wavefunction, denoted CISD[TQ], has previously been shown to recover a large fraction of the energy of a CI wavefunction including all single, double, triple, and quadruple excitations (CISDTQ). A comparison of the molecular geometry and harmonic vibrational frequencies of hydrogen sulfide (H2S) predicted by two CISD[TQ] wavefunctions and the complete CISDTQ wavefunction are presented. With the largest basis set used, a triple-zeta plus double polarization basis with an additional set of d-type functions added to hydrogen, and an additional set of f-type functions added to sulfur [TZ2P(f,d)], the CISD[TQ] predictions differ from the CISDTQ by 0.0003 Ang in the bond length and by 0.02o in the bond angle. The CISD[TQ] harmonic vibrational frequencies differ by less than 2 cm-1 from the full CISDTQ predictions. These results suggest that the CISD[TQ] wavefunction is an efficient and accurate truncation of the complete CISDTQ and are particularly impressive considering that with a TZ2P(f,d) basis, the larger CISD[TQ] wavefunctions included roughly 300,000 configurations while the CISDTQ includes almost nine million.