Self-Consistent Molecular Orbital Methods. V. Ab Initio Calculation of Equilibrium Geometries and Quadratic Force Constants

Ab initio calculation of equilibrium geometries and quadratic force constants for a large group of first-row polyatomic molecules has been carried out, using the previously described J. Chem. Phys. 51, 2657 (1969) STO–3G approximation for STO basis functions. The average deviation of calculated and experimental bond lengths and angles is 0.035 Å and 1.7°, respectively. Nearly all important experimental trends are reproduced. For a few cases involving bonds between electronegative atoms, significant discrepancies are found. Quadratic force constants are evaluated for symmetric stretching and bending modes and are found to be overestimated, typically by 20%–30%. Nearly all experimental trends are satisfactorily accounted for. It is concluded that a minimal STO basis with properly chosen orbital exponents offers a useful and computationally efficient model for potential surface studies.