Keyword category:

Configuration Interaction / Spin State

SCFCI=n.nnnn

Description

Defines the partial occupancy of the "virtual" CI-active MOs for a "half-electron" RHF SCF calculation done prior to a CI calculation. These occupancies will be borrowed from occupied CI-active MOs. SCFCI=n.nnnn does not define or assume an active space for the CI, so a reasonable one must be specified using OPEN(n,m) and/or one of the explicit CI keywords. For example, if the defined active space consists of 3 doubly occupied MOs (6 electrons) and 4 virtual MOs (e.g., C.I.(6,12), where 8 is the HOMO)and SCFCI=0.4 is specified, then during the "half-electron" RHF SCF calculation prior to the CI, the occupancies of the CI-active MOs will be (6 - 4 × (0.4)) / 3 = 1.46666 for the previously doubly-occupied MOs and 0.4 for the previously virtual MOs. If the keyword SCFCI is specified without an assigned value, then a default value for n.nnnn is calculated as a / (2b(c - b) + 1), where a is the number of CI-active electrons, b is the number of occupied CI-active MOs and c is the total number of CI-active MOs. In the above example of 3 doubly-occupied CI-active MOs and 4 virtual CI-active MOs, the default value for n.nnnn would be 6 / (2 * 3(7 - 3) + 1) = 0.24. If there are both doubly-occupied and fractionally-occupied CI-active MOs defined, then their occupancies after borrowing will be constrained to be equal for the "half-electron" RHF SCF. Also, note that the occupancies of the "virtual" CI-active MOs cannot be defined to be greater than that of the "occupied" CI-active MOs.

See also:

C.I., OPEN.