``Molecular Geometry and Vibrational Frequency Predictions from the CISD[TQ] Wavefunction: The Water Molecule,'' Rollin A. King, C. David Sherrill, and Henry F. Schaefer III, Spectrochimica Acta Part A, 106, 1163 (1997).

A highly compact and accurate configuration interaction wavefunction including all single and double excitations and certain triple and quadruple excitations selected in an a priori fashion, called the CISD[TQ] wavefunction, has previously been found to provide energies very near those of the configuration interaction wavefunction including all single, double, triple, and quadruple excitations (CISDTQ). This work reports the CISD[TQ] wavefunction predictions for the molecular geometry and the harmonic vibrational frequencies of H2O in comparison to the predictions of the complete CISDTQ wavefunction. With a triple-zeta plus double polarization (TZ2P) basis, the geometry predicted by the CISD[TQ] wavefunction differs from that of the CISDTQ by less than 0.0001 A in the bond length and 0.2 deg in the bond angle. The harmonic vibrational frequencies of the CISD[TQ] method differ from the complete CISDTQ by an average of only 5 wavenumbers. With this basis the complete CISDTQ wavefunction includes 45 times the number of configuration state functions as the CISD[TQ] wavefunction. These encouraging results suggest that the CISD[TQ] wavefunction can be an efficient, accurate truncation of the complete CISDTQ.