6.15 Defining orbital subspaces

In the SCF, MCSCF, and CI programs it may be necessary to specify how many orbitals in each symmetry are occupied (or internal in CI), and which of these are core or closed shell (doubly occupied in all CSFs). This information is provided on the OCC, CORE, and CLOSED cards in the following way:

OCC, $m_1,m_2,\ldots,m_8$; CORE, $co_1,co_2,\ldots,co_8$; CLOSED, $cl_1,cl_2,\ldots,cl_8$;

where $m_i$ is the number of occupied orbitals (including core and closed), $co_i$ the number of core orbitals, and $cl_i$ is the number of closed-shell orbitals (including the core orbitals) in the irreducible representation $i$. In general, $m_i \ge cl_i$, and $cl_i \ge co_i$. It is assumed that these numbers refer to the first orbitals in each irrep.

Note that the OCC, CORE, and CLOSED cards have slightly different meanings in the SCF, MCSCF and CI or CCSD programs. In SCF and MCSCF, occupied orbitals are those which occur in any of the CSFs. In electron correlation methods (CI, MPn, CCSD etc), however, OCC denotes the orbitals which are occupied in any of the reference CSFs. In the MCSCF, core orbitals are doubly occupied in all CSFs and frozen (not optimized), while closed denotes all doubly occupied orbitals (frozen plus optimized). In the CI and CCSD programs, core orbitals are those which are not correlated and closed orbitals are those which are doubly occupied in all reference CSFs.

OCC, CORE and CLOSED commands are generally required in each program module where they are relevant; however, the program remembers the most recently used values, and so the commands may be omitted if the orbital spaces are not to be changed from their previous values. Note that this information is also preserved across restarts. Note also, as with the WF information, sensible defaults are assumed for these orbital spaces. For full details, see the appropriate program description.