#### Projected Seniority-Two Orbital Optimization of the Antisymmetric Product of One-Reference Orbital Geminal

**K Boguslawski**, P Tecmer, PA Limacher, PA Johnson, PW Ayers, P Bultinck, S De Baerdemacker, D Van Neck

Journal PaperThe Journal of Chemical Physics 140 (21), 214114 (**2014**)

#### 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 C_{2} and LiF molecules, for the CH_{2} 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.