| Abstract
| - The perfect pairing (PP) approximation from generalized valence bond theory is formulated in an unrestrictedfashion for both closed- and open-shell systems using a coupled cluster ansatz. In the model chemistry proposedhere, active electron pairs are correlated, but the unpaired or radical electrons remain uncorrelated, leading toa linear number of decoupled cluster amplitudes which can be solved for analytically. The alpha and betaspatial orbitals are variationally optimized independently. This minimal treatment of electron−electroncorrelation noticeably improves upon symmetry-breaking problems and other pathologies in Hartree−Fock(HF) theory and may be computed using the resolution of the identity approximation at only a factor ofseveral times more effort than HF itself. PP also generally predicts improved molecular structures over HF.This compact, correlated wave function potentially provides a useful starting point for dynamical correlationcorrections.
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