The Spin Multiplicity window allows you to select only physically meaningful
combinations of open shells and multiplicity. To select a combination of spin
multiplicity/no. of open shells, you must click on the diamond corresponding to your
choice.
Note that in some cases portions of the spin diagram below the diagonal may
be disabled, even though these states are physically meaningful. The reason
for this is that most open shell Hartree-Fock (HF) programs are limited to
handling only high spin states. For example, if there were 4 open shells in
a molecule, most HF programs would be able to handle only the quintet state
(not the triplet or singlet). In order to access the triplet and singlet
states, it would be necessary to use an HF program that allowed such states,
or resort to a Generalized Valence Bond (GVB) theory, instead of ROHF.
Where there are multiple spin states possible (e.g., there are 3 ways of
constructing a triplet state with 4 open shells), you will have to input
spin coupling coefficients in order to have the GVB code solve for the
correct state.
