Unfortunately, much does depend on the particular task and chemical intuition; experimentation with the active space may be quite advisable (such as its gradual expansion). Bond breaking reactions would definitely require inclusion of the respective bonding and antibonding orbitals but more frontier orbitals may be relevant as well. To reduce the amount of "mess", one may request orbital localization and assemble the initial approximation to the active space from the localized orbitals. Have you seen this review: http://www.annualreviews.org/doi/abs/10.1146/annurev.physchem.49.1.233 ?
I guess you are interested in the ground state. However, as Pavlo has pointed out, several states may come close, so you may need to select some reasonable value of NSTATE (not exceedingly large to spare computational time) and, if you really have a closely spaced ensemble of states to test the effects of state averaging via WSTATE. If the two states are of different symmetry and do not exhibit interaction via symmetry breaking, one may prefer to consider them separately; however, an averaged calculation would provide a more balanced description of their relative energy. One may also try to optimize more than one state using state tracking: the states can reorder depending on the geometry. To verify the adequacy of CASSCF, single point XMCQDPT2 tests can be helpful.
On Thu Sep 11 '14 5:05am, alex wrote
>Please disregard my questions #3 and #4; any comments on #1 and #2 are still welcome.
>On Wed Sep 10 '14 3:19am, alex wrote
>>Would you please share some ideas of treating of protein-substrate complex with mcscf method? I have some problems with setting up of active space, maybe you can advise something:
>>1. Is it possible to print transposed (and even better then sorted) matrix of eigenvectors as well? It’s not a problem to find interesting molecular orbitals if the system consists of the only few but the selection is just overwhelming if there are thousands orbitals. My life would be much easier if firefly highlight the several orbital to consider for the active space instead of all that mess.
>>2. How to suggest amount of states (NSTATE) and their weights (WSTATE) from knowledge about the chemical reaction? How it is related to the active space?
>>3. How to carry out geometry optimization for mcscf? As I understand the orbitals will change at each step of the search. But I can reorder/select them only at the initial configuration… How the correct active space is determined/maintained during geometry search?
>>4. My system is oxidase, i.e. triplet. But if I am trying to set MULT=3 and NAOS=2 NBOS=0 it complains “NAOS.NE.NBOS IN DRT INPUT DOES NOT MAKE SENSE.” How to model the triplet then and why there is no sense?