title: Revisitation of Nonorthogonal Spin Adaptation in Coupled Cluster Theory.
abstract: The benefits of what is alternatively called a nonorthogonally spin-adapted, spin-free, or orbital representation of the coupled cluster equations is discussed relative to orthogonally spin-adapted, spin-orbital, and spin-integrated theories. In particular, specific linear combinations of the orbital cluster amplitudes, denoted spin-summed amplitudes, are shown to reduce the number of contractions that must be explicitly performed and to simplify the expressions and their derivation. The computational efficiency of the spin-summed approach is discussed and compared to orthogonally spin-adapted and spin-integrated approaches. The spin-summed approach is shown to have significant computational advantages relative to the other methods for CCSDT and CCSDTQ.
authors.affiliation: Department of Chemistry and Biochemistry, Institute for Theoretical Chemistry, The University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: Devin A
authors.initials: DA
authors.lastname: Matthews
authors.affiliation: Institut für Physikalische Chemie, Universität Mainz , D-55099 Mainz, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Gauss
authors.affiliation: Department of Chemistry and Biochemistry, Institute for Theoretical Chemistry, The University of Texas at Austin , Austin, Texas 78712, United States.
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton