Multiconfiguration perturbation theory with imaginary level shift

In multiconfigurational perturbation theory, so-called intruders may cause singularities in thr potential energy functions at geometries where an energy denominator becomes zero. When the singularities are weak, they may be successfully removed by level shift techniques. When applied to excited states a small shift merely moves the singularity. A large shift may cause new divergencies, and too large shifts are unacceptable since the potential function is affected in regions further away from the singularities. This Letter presents an alternative which may be regarded as an imaginary shift. The singularities are not moved but disappear completely. They are replaced by a small distortion of the potential function. Applications to the N-2 ground state, its A(3) Sigma(u)(+) state, and the Cr-2 ground state how that the distortion caused by this procedure is small. (C) 1997 Elsevier Science B.V.