SCFCI — Defines two sets of open-shell MOs and their fractional occupancies to be used in a “Half-Electron” RHF SCF calculation preceding a CI calculation.
Defines two sets of open-shell MOs and their occupancies for a “Half-Electron” RHF SCF calculation to be done prior to a CI calculation in which at least those MOs will be CI-active. SCFCI is a generalization of the OPEN(n,m) keyword. The total number of open-shell electrons is defined by the n parameter, which must be even for a molecule with an even number of electrons and odd for a molecule with an odd number of electrons. These n electrons are distributed among m1+ m2 MOs, which are divided into two sets consisting of m1 MOs and m2 MOs, respectively. The m1+ m2 MOs to be used are chosen in order of increasing energy starting from the group of highest energy occupied MOs sufficient to contain n electrons if the MOs were AufBau-occupied according to a low-spin ROHF scheme. The occupancies of the m1 MOs in the first set are different from the m2 MOs in the second set, but within each set the MO occupancies are the same. The ratio of the occupancy of an MO in the second set to that of an MO in the first set is defined by the parameter r, where 0.0 < r < 1.0. Thus, the occupancy of each of the m1 MOs will be (nm1)/(m1+rm2) while the occupancy of each of the m2 MOs will be (nrm2)/(m1+rm2).
For example, specifying SCFCI(2,2,2,0.5) for a molecule with an even number of electrons will result in 2 electrons distributed among 4 open-shell MOs, with two of the MOs having an occupancy of 0.666667 and corresponding to the HOMO and LUMO of an RHF calculation and with the other two MOS having an occupancy of 0.333333 and corresponding to the LUMO+1 and LUMO+2 MOs of an RHF calculation. For a molecule with an odd number of electrons, specifying SCFCI(3,2,2,0.5) will result in 3 electrons distributed among 4 open-shell MOs, but with two of the MOs having an occupancy of 1.0 and corresponding to the HOMO-1 and HOMO of a ROHF doublet calculation and with the other two MOS having an occupancy of 0.5 and corresponding to the LUMO and LUMO+1 MOs of an ROHF doublet calculation.
If the SCFCI keyword is specified without an explicit CI keyword, then following the open-shell RHF SCF calculation, a CAS CI calculation will be done using the set of m1 + m2 MOs as the CI-active MOs. If an explicit CI keyword is specified along with SCFCI, then the CI calculation will be done accordingly, but the active space of MOs must include at least the set of m1 + m2 MOs.
If the fractional occupancy ratio of the two sets of MOs is not specified via the r parameter, then a reasonable default value will be assigned by AMPAC.
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