OPEN — Configuration Interaction
Specifies that the SCF calculation should be done using the “half-electron” method, with n electrons distributed equally among m MOs, which are filled after the doubly occupied MOs. These open-shell MOs have an occupancy of (n / m) while the corresponding SOs have occupancies of (n / 2m). This cannot be used with UHF.
This keyword is compatible with ROHF as long as n=m, so that the open-shell occupancy is 1.00.
For the RHF case if CI is not also explicitly invoked, a CAS-CI is done in which the
fractionally occupied MOs define the active space. Otherwise, the specified type of CI will be
done, but care should be used in defining the active space so that all of the partially
occupied MOs are included. For odd-electron systems, OPEN(1,1) is implied if the
n,m) keyword is not specified. If
EXCITED is specified,
OPEN(2,2) is implied.
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