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Reference Function

Calculations can be carried out with the following choices of reference functions (keyword REF=reference)

closed-shell cases

REF=RHF is here the default, REF=UHF might be required for spin-symmetry broken calculations as well as the computation of some spin-dependent properties such as for example the FC and SD contribution to indirect spin-spin coupling constants-

high-spin open-shell cases

The available choices are here either REF=UHF or REF=ROHF (note that for these cases the reference function must be explicitly specified as well as the corresponding multiplicity mult via the keyword MULT=mult

low-spin open-shell cases

For low-spin open-shell singlets, the option REF=ROHF (non-totally symmetric states) is available. Note that this option only allows SCF and Mk-MRCC calculations.

multi-configurational cases

At the multi-configurational level, the only avaiable option is REF=TCSCF (two-configurational SCF for totally symmetric states) for cases where two closed-shell configurations are required for an adequate description. Note that this option can be only used in SCF and Mk-MRCC calculations.

As additional possibility (particular for open-shell states), the so-called QRHF reference can bs used (see QRHF-CC calculations).