title: Projected seniority-two orbital optimization of the antisymmetric product of one-reference orbital geminal.
abstract: We present a new, non-variational orbital-optimization scheme for the antisymmetric product of one-reference orbital geminal wave function. Our approach is motivated by the observation that an orbital-optimized seniority-zero configuration interaction (CI) expansion yields similar results to an orbital-optimized seniority-zero-plus-two CI expansion [L. Bytautas, T. M. Henderson, C. A. Jimenez-Hoyos, J. K. Ellis, and G. E. Scuseria, J. Chem. Phys. 135, 044119 (2011)]. A numerical analysis is performed for the C2 and LiF molecules, for the CH2 singlet diradical as well as for the symmetric stretching of hypothetical (linear) hydrogen chains. For these test cases, the proposed orbital-optimization protocol yields similar results to its variational orbital optimization counterpart, but prevents symmetry-breaking of molecular orbitals in most cases.
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, Hamilton, 1280 Main Street West, Ontario L8S 4M1, Canada.
authors.forename: Katharina
authors.initials: K
authors.lastname: Boguslawski
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, Hamilton, 1280 Main Street West, Ontario L8S 4M1, Canada.
authors.forename: Paweł
authors.initials: P
authors.lastname: Tecmer
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, Hamilton, 1280 Main Street West, Ontario L8S 4M1, Canada.
authors.forename: Peter A
authors.initials: PA
authors.lastname: Limacher
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, Hamilton, 1280 Main Street West, Ontario L8S 4M1, Canada.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Johnson
authors.affiliation: Department of Chemistry and Chemical Biology, McMaster University, Hamilton, 1280 Main Street West, Ontario L8S 4M1, Canada.
authors.forename: Paul W
authors.initials: PW
authors.lastname: Ayers
authors.affiliation: Department of Inorganic and Physical Chemistry, Ghent University, Krijgslaan 281 (S3), 9000 Gent, Belgium.
authors.forename: Patrick
authors.initials: P
authors.lastname: Bultinck
authors.affiliation: Center for Molecular Modelling, Ghent University, Technologiepark 903, 9052 Gent, Belgium.
authors.forename: Stijn
authors.initials: S
authors.lastname: De Baerdemacker
authors.affiliation: Center for Molecular Modelling, Ghent University, Technologiepark 903, 9052 Gent, Belgium.
authors.forename: Dimitri
authors.initials: D
authors.lastname: Van Neck