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# 7.12.7 Spin-flip CAS with Perturbative External Singles Corrections (casman)

(February 4, 2022)

The casman module allows one to run spin-flip CAS-based calculations, with perturbative corrections accounting for hole and particle excitations outside of the active space. (For variational hole and particle excitations, please refer to the section on librassf.) It also allows one to run with a limited number of spin-flips, which can be useful for large transition metal complexes in which the maximum number of spin-flips cannot be performed. For more details on this approach, please refer to the publication by Mayhall and Head-Gordon.766

Q-Chem has several built-in excitation schemes. To use these, simply set CORRELATION to the appropriate value. The following methods are currently available: SF-CAS, SF-CAS(h,p), SF-CAS(S), SF-CAS(2x), SF-CAS(3x), and SF-CAS(3x)+D.

Alternatively, if a custom method is desired, one can set CORRELATION = SF-CAS and manually define the appropriate \$rem variables as needed. Please see the section on casman job control options for further details.