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absorption/emission MCSCF (again)

Jonas Baltrusaitis

I have a broad question to all the MCSCF wizzes. My problem is simple and repetitive in this forum: getting absorption/emission spectra. It's painfully clear from this forum what needs to be done:
1. optimize ground state MCSCF (or simple DFT), calculate excited state spectra with MCSCF (or higher multiconfigurational methods, if they work in parallel).
2. pick the brightest state, optimize its geometry with MCSCF, calculate excited state spectar again. here, however, simple TDDFT is impossible (no gradients in FF) or might be inacurate for trickier bond breaking/multiconfigurational solutions, CI might also fail. so geometry optimization with MCSCF is only reasonable solution
I will put forth that by now I know how to setup MCSCF input via CI calculations, orbital reordering etc.

My biggest worry here is that I have no idea how to pick up CAS space for any inocuos molecule, e.g. something that does not have well known electronic transitions as in ample small molecule examples circulating in forum. Do I look at natural orbital occupation only in this case for values between ~1.9 and ~0.2? Looking at orbital shape in a large molecule almost doesn't matter (if it's not a bond breaking situation) as it doesn;t provide any information as those orbitals will hybridize over many atoms thus straying away from awell known conceptual orbitals.

I have attached a typical molecule that of absorption/emission situation and no particular well known electronic transitions, maybe Alex or somebody else could comment on how to pick CAS space here

State averaging is also almost loosely defined. It's not very clear why/when would do state averaging...



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