CASSCF-NMR, active space for compound and reference

[miro_ilias]

Hello,

my diploma students are calculating CASSCF-NMR shieldings, and we would be glad to clarify setting the active space of the studied molecule and of the (smaller) reference molecule.

It seems to me fine to use the same number of active electrons and the same active space for both the molecule and the reference.

Any ideas are welcome.

Miro

[hjaaj]

Dear Miro, one cannot answer your question without knowing more about the molecule and the reference molecule.

[miro_ilias]

Hi,

so in general it is larger molecule and typical NMR reference, smaller molecule, like TMS, H3PO4. In my case it is sarin/THC and the NMR reference - TMS.

[hjaaj]

Tjah. I would do MP2 calculations first and look at the MP2 natural orbital occupation numbers and use these as a guide for the selection of active spaces. It would be pure coincidence if a good choice of active space was the same size for the two molecules.

It is important that the choice of active orbitals is balanced across symmetries if the molecule has symmetry, otherwise one will probably experience instabilities in the response calculations. Unbalanced is for example if one includes an orbital with MP2 predicted occupation number of e.g. 1.96, but does not include another orbital with 1.94 predicted.