FILL — Require use of defined set of prototype MOs.
After each SCF iteration MOs are usually filled based on increasing MO energy using the Aufbau principle. In the course of a reaction or geometry optimization, the MO description may change substantially. The FILL keyword and its variants are intended to give the user some control over this situation. Use of FILL requires that the SCF calculations be done using a prototype set of MOs as a reference when deciding the occupancy of the current SCF MOs. After the first SCF iteration (not after the first full SCF calculation, but after the first iteration within the first SCF calculation), the full set of occupied MOs determined by the Aufbau method are defined as prototypes for deciding the MO occupancies after the next SCF iteration. The resulting occupied MOs then become prototypes for the next SCF iteration, and so on. These evolving prototype MOs thus remain filled for both the remainder of the first SCF calculation as well as any subsequent SCF calculations (done at different geometries during geometry optimizations, for example). Least squares overlap is used to determine the similarity of the current SCF MOs and the prototype MOs in order to select those which most closely correspond to the prototypes. FILL and its variants are functional only with RHF calculations, whether open-shell or closed-shell.
When this keyword is specified, the nth MO after the first SCF iteration is constrained to be the last MO filled with electrons, i.e., the HOMO, and this MO will be used as an evolving prototype for the HOMO in all subsequent SCF iterations. FILL and its variants are functional only with RHF calculations, whether open-shell or closed-shell.
When this keyword is specified, the MOs with the specified indices n1, n2, etc. after the first SCF iteration are constrained to be the last MOs filled with electrons, and they will be used as a evolving prototypes for the last filled electrons in all subsequent SCF iterations. The order of the MOs in the argument list is important for open-shell systems, in that those specified last are the partially filled MOs. FILL and its variants are functional only with RHF calculations, whether open-shell or closed-shell.
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