Posted by Per Åke Malmqvist on October 18, 2004 at 08:41:02:
In Reply to: RASSCF output question posted by Sergiu Clima on October 15, 2004 at 13:44:23:
Dear Sergiu,
The notation refers to a sequential spin coupling using UGA phase rules.
Ex: 20uudd means:
Take two electrons in active orbital 1,
Add zero electrons, in active orbital 2,
Add a single electron in active orbital 3, coupled up, etc.
Two or zero electrons form a singlet, so their coupling is trivial.
Coupling up means that the spin is raised 1/2 unit, etc.
From spin 0, one CANNOT couple down!
So for instance, with four electrons in four orbitals, one could have the
two CSF's uudd or udud, but never uddu,duud,dudu,or dduu. For the same reason,
one cannot have 000020dduu.