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Updated: Mar 30, 2026

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
Revisitation of Nonorthogonal Spin Adaptation in Coupled Cluster Theory
Devin A Matthews1, Jürgen Gauss2, John F Stanton1
1Department of Chemistry and Biochemistry, Institute for Theoretical Chemistry, The University of Texas at Austin , Austin, Texas 78712, United States.
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.
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