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Добромир Антонов Калчевски
dobromirak@gmail.com

Is it possible, in the case of CASSCF, to correctly transit a calculation from a smaller to a larger basis set, and by correctly - I mean as Active Space. Considering I have to reorder certain MOs for the calculations with the smaller basis, will I get the correct MOs from the larger one, without new reordering ? Also, since basis sets with diffusive and polarizing functions tend to substitute MOs with not only more, but qualitatively different MOs, which is more visible in the Vacant part, am I to expect that a sufficient active space can be sustained at a larger basis. What would be the way to do it, in terms of correct calculation steps and keywords. Let's say calculating transitions for excited states will give all sorts of differences in the selection of the active MOs, depending on the basis.

Can I do an extrapolation such as STO-3G -> 4-31g -> 6-31g -> 6-31g* -> cc-pVTZ without polluting the active space with orbitals, which are not supposed to be a part of it, AND without losing any of the ones which should be a part.

Also, is there a significant improvement of time requirements if I do this trick or not ? I know a larger basis will converge with less steps. Is there a chance of qualitative difference so big, that my calculation can even fail under a smaller basis set ? Let's say I'm searching for a Conical Intersection of a molecule with close to 20 2nd row atoms + close to 30 hydrogen atoms, and quite conjugated, at least at Ground State.

Also what would be the output of Firefly 810, in case there is no conical intersection ? Roughly said, as a number of steps or proportion of time (compared to a case with conical intersection) am I for a long wait ?

My computer is an i7 3770k, 16GB. I have access to another one - x5650, 98GB. Can someone with a similar experience give me some kind of an estimate of the time in front of me, if I've picked let's say 8el 8orb active space, and it happens to be correct.

Thank you in advance,
Dobromir