title: Unrestricted perfect pairing: the simplest wave-function-based model chemistry beyond mean field.
abstract: The perfect pairing (PP) approximation from generalized valence bond theory is formulated in an unrestricted fashion for both closed- and open-shell systems using a coupled cluster ansatz. In the model chemistry proposed here, active electron pairs are correlated, but the unpaired or radical electrons remain uncorrelated, leading to a linear number of decoupled cluster amplitudes which can be solved for analytically. The alpha and beta spatial orbitals are variationally optimized independently. This minimal treatment of electron-electron correlation 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 of several 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 correlation corrections.
authors.affiliation: Department of Chemistry, University of California, and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460, USA.
authors.forename: Gregory J O
authors.initials: GJ
authors.lastname: Beran
authors.forename: Brian
authors.initials: B
authors.lastname: Austin
authors.forename: Alex
authors.initials: A
authors.lastname: Sodt
authors.forename: Martin
authors.initials: M
authors.lastname: Head-Gordon