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Open-shell CC Treatment

For open-shell molecules, several options exist which differ in the choice of reference functions and in the treatment of spin.

Standard choice is a spin-orbital CC treatment using either a UHF or ROHF reference functions and either of these two possibilities are recommended for routine usage.

Other options concerning reference functions include the usage of a QRHF reference functions (see above). Concerning the treatment of spin, either a partical spin-adapted CC or a spin-restriced CC treatment is possible. These methods are invoked by

OPEN-SHELL=PSA-CC (invokes a partially spin-adapted CCSD calculation)

OPEN-SHELL=SR-CC (invokes a spin-restricted CCSD calculation)

The (recommended) default choice is OPEN-SHELL=SPIN_ORBITAL