The imperfect pairing approximation

We present a wavefunction intended to model the static correlation of molecular systems. This wavefunction is best understood as a generalization of the perfect pairing (PP) approximation, and it is therefore termed the imperfect pairing (IP) approximation. The energy is determined by exploiting the connection between PP and a constrained coupled-cluster approach, and optimizing the orbitals to minimize the energy. We obtain results for some trial systems and find that, while a larger fraction of the correlation energy is recovered in IP than PP, the newer method has certain systematic limitations. (C) 2000 Elsevier Science B.V. All rights reserved.