Dear all,
I am seeking advice on how to construct an excited state (for example, by swapping HOMO/LUMO orbitals) and how to maintain the order of orbitals within the SCF procedure and during geometry optimisation. While there is an option in the orbital setup to flip the spin (e.g. to generate start MOs for a broken symmetry treatment), there does not seem to be an option to swap orbitals.
Specifically, I wish to construct an excited state for an open-shell TM species by exclusively swapping the SOMO and SOMO+1 orbitals within the beta manifold (such a state would describe a formal electron transfer between parts of the TM molecule; for example, a Co(III) porphyrin interacting with a carboradical versus a Co(II) porphyrin interacting with a carbocation). This electronic configuration should be maintained using a conventional DFT method and also during geometry optimisation. As there is no orbital swapping option available in DEFINE, I adapted the beta file manually to perform the swap. As a last resort, it was even tried changing the occupation of the beta electrons; let’s say from "a 1-252" to "a 1-251, 253", but this did not work. The following options were tried to maintain the excited state configuration during the SCF procedure in RIDFT:
(1) $scfdamp start=5.500 step=0.050 min=0.500
$scforbitalshift automatic=0.10
(2) $fermi tmstrt=300.00 tmend=150.00 tmfac=0.98 hlcrt=1.0E-01 stop=1.0E-03 nue=1
Unfortunately, none of these options achieves the desired result. The hand-constructed excited configuration collapses more or less quickly into the initial state before the SOMO/SOMO+1 swap is made within the beta manifold. Furthermore, treatment by ESCF does not appear to work either.
Has anyone encountered similar problems when treating a formal electron transfer on an open-shell TM complex (such as the aforementioned two Co-porphyrin species with an identical electron count) using a conventional DFT single-determinant method? I am well aware that CASSCF/NEVPT2 would be the method of choice, but it is too expensive.
Any advice would be much appreciated.
Thanks, Paul
