MULTICONFIGURATION PERTURBATION-THEORY WITH IMAGINARY LEVEL SHIFT

In multiconfigurational perturbation theory, so-called intruders may c ause singularities in thr potential energy functions at geometries whe re an energy denominator becomes zero. When the singularities are weak , they may be successfully removed by level shift techniques. When app lied to excited states a small shift merely moves the singularity. A l arge shift may cause new divergencies, and too large shifts are unacce ptable since the potential function is affected in regions further awa y from the singularities. This Letter presents an alternative which ma y be regarded as an imaginary shift. The singularities are not moved b ut disappear completely. They are replaced by a small distortion of th e potential function. Applications to the N-2 ground state, its A(3) S igma(u)(+) state, and the Cr-2 ground state how that the distortion ca used by this procedure is small. (C) 1997 Elsevier Science B.V.