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).