Direct vibrational self-consistent field method:: Applications to H2O and H2CO

The vibrational self-consistent field (VSCF) and virtual configuration interaction (VCI) methods are directly combined with ab initio electronic structure calculations for evaluations of the potential energy at VSCF quadrature points. Referred to as direct VSCF and direct VCI, respectively, these methods have been applied to evaluations of anharmonic vibrational energy levels of H2O and H2CO at the second-order Moller-Plesset MP2/aug-cc-pVTZ and MP2/cc-pVTZ computational levels, respectively. The purpose of the present study is to develop a direct methodology for vibrational state calculations by examining the accuracy of the results, as well as their computational costs. In addition, the accuracy and applicability of two approximate potential energy surfaces (PES), a quartic force field (QFF), and the PES determined by the modified-Shepard interpolation method (Int-PES), are investigated via comparisons of calculated energy levels of vibrational states with those derived by the direct methods. The results are analyzed in terms of three considerations: (i) truncations of higher-order intercoordinate couplings in the PES; (ii) mode-mode coupling effects; (iii) approximations in ab initio electronic structure methods. In the direct VCI calculations, the average absolute deviations in fundamental frequencies relative to the experimental values are 9.3 cm(-1)(H2O) and 34.7 cm(-1)(H2CO). The corresponding values evaluated with approximate PESs relative to those derived by the direct method are 35.0 cm(-1) (QFF) and 15.3 cm(-1) (Int-PES) for H2O, and 6.3 cm(-1) (QFF) and 10.3 cm(-1) (Int-PES) for H2CO. (C) 2000 American Institute of Physics. [S0021-9606(00)30427-5].