HARTREE-FOCK ORBITAL INSTABILITY ENVELOPES IN HIGHLY CORRELATED SINGLE-REFERENCE WAVE-FUNCTIONS

The effects of Hartree-Fock orbital instabilities on force constant pr edictions at both Hartree-Fock and correlated levels of theory are inv estigated. Due to the quadratic dependence of the second derivative of correlated energies on the orbital rotation parameters, anomalous for ce constant singularities enveloped by ''instability volcanoes'' are g iven by the single-reference correlation methods examined here. Infini te-order coupled-cluster methods are indeed affected by the reference instability, but over a rather small region of the potential surface, whereas perturbative triples corrections tend to widen the coupled-clu ster volcano. Finite-order many-body perturbation theory yields very w ide volcanoes, and corresponding predictions of vibrational spectra ma y be seriously compromised if the geometry of interest lies at all in the vicinity of an instability in the reference determinant. (C) 1997 American Institute of Physics.