title: Second-Order Perturbation Theory for Fractional Occupation Systems: Applications to Ionization Potential and Electron Affinity Calculations.
abstract: Recently, we have developed an integration approach for the calculations of ionization potentials (IPs) and electron affinities (EAs) of molecular systems at the level of second-order Møller-Plesset (MP2) (Su, N. Q.; Xu, X. J. Chem. Theory Comput. 11, 4677, 2015), where the full MP2 energy gradient with respect to the orbital occupation numbers was derived but only at integer occupations. The theory is completed here to cover the fractional occupation systems, such that Slater's transition state concept can be used to have accurate predictions of IPs and EAs. Antisymmetrized Goldstone diagrams have been employed for interpretations and better understanding of the derived equations, where two additional rules were introduced in the present work specifically for hole or particle lines with fractional occupation numbers.
authors.affiliation: Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University , Shanghai 200433, China.
authors.forename: Neil Qiang
authors.initials: NQ
authors.lastname: Su
authors.affiliation: Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, MOE Laboratory for Computational Physical Science, Department of Chemistry, Fudan University , Shanghai 200433, China.
authors.forename: Xin
authors.initials: X
authors.lastname: Xu