The n'th M.O. in an RHF calculation is constrained to be filled. It has no effect on a UHF calculation. After the first iteration (NOTE:   not after the first SCF calculation, but after the first iteration within the first SCF calculation) the n'th M.O. is stored, and, if occupied, no further action is taken at that time. If unoccupied, then the HOMO and the n'th M.O.s are swapped around, so that the n'th M.O. is now filled. In all subsequent iterations the M.O. nearest in character to the stored M.O. is forced to be filled, and the stored M.O. replaced by that M.O. This is necessitated by the fact that in a reaction a particular M.O. may change its character considerably. A useful procedure is to run 1SCF and DENOUT first, in order to identify the M.O.s. The complete job is then run with OLDENS and FILL=nn, so that the eigenvectors at the first iteration are fully known. As FILL is known to give difficulty at times, consider using using C.I.=n and ROOT=m instead.