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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 $ p^5 s^1$ $ ^3$P states, or the $ d^4s^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:

$\displaystyle a =$ $\displaystyle 1$   always    
$\displaystyle b =$ $\displaystyle 2$   if$\displaystyle \; n \le n_{ir} \qquad b = \frac{(2n_{ir})}{n}$   if$\displaystyle \; n > n_{ir}$    

where $ n_{ir}$ and $ n$ refer to shell 1.