Projected seniority-two orbital optimization of the Antisymmetric   Product of one-reference orbital Geminal
  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 [J. Chem. Phys., 135, 044119 (2011)]. A numerical analysis is performed for the C$_2$, LiF and CH$_2$ molecules 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. 
Katharina Boguslawski and Pawe{\l} Tecmer and Peter A. Limacher and   Paul A. Johnson and Paul W. Ayers and Patrick Bultinck...
cond-mat.str-el physics.chem-ph