Two open shells

This becomes tricky in general and we give only the most important case:

shell 1

is a Roothaan case, see 6.3.2

shell 2

is one electron in an a (s) MO (n_ir = 1)

with parallel spin coupling of shells.

This covers e.g. the p⁵ s^{1 3}P states, or the d⁴s^{1 6}D states of atoms. The coupling information is given following the keyword $rohf. The (a, b) within a shell are taken from above (6.3.2), the cross term (shell 1)-(shell 2) is in this case:

a = 1    always

$${\frac{{(2n_{ir})}}{{n}}}$$ b =

where n_ir and n refer to shell 1.