Open-Shell Orthogonal Molecular Orbital Theory

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Abstract
Computationally convenient formulas are derived for the matrix elements of the identity and of one‐ and two‐electron operators, for open‐shell spin‐projected wavefunctions built from orthogonal spatial orbitals. These formulas explicitly show the coefficients of the one‐ and two‐electron integrals involved, and are not much more complicated than the closed‐shell formulas in common use. Results are presented in tabular from for all types of configuration pairs possessing nonzero matrix elements, and examples of the use of the formalism are given. These results should facilitate the development of open‐shell extended Hartree—Fock theory.