CASSCF questions

["Varnai Peter" <VARNAI %-% at %-% ch.bme.hu>]

 Dear CClers,
 I would need Your help to understand the practical tricks in
 CASSCF calculations.
 1, One needs to define an active space.
         -At this point quantum chemists start to think as the
         conventional organic chemist,in localized orbitals. Here
         those orbitals needed -in view of a chemist- that are
         involved in a eg. reaction (forming/breaking orbitals).
         -A more exact answer would be that those orbitals needed
          which are important in "excitation" to give configurations
          that contribute to the total energy significantly.
 So: -according to the first approach one would visualize the MOs.
 Here he/she would realize that in MO theory the orbitals are spread
 all over the molecule, so the needed eg C-C bond will appear in
 at least 3 MOs. Which one should be used then? Let's use all
 (unfortunately, the valence MOs  usually can't be all involved in
 the active space)? Let's localize the MOs with standard methods?
 Pick one random and what happens (the one is better than the other if
 using it the SCF energy is lower)?
     -according to the more "mathematical" approach let's calculate a
 CISD wavefunction  [in the absence of the full CI :(]  and involve
 the orbitals which are involved in large coefficient (say 0.03<)
 determinants? And what if the HF reference function was not a good
 approximation and some important determinants are missed?
      -If I am right CAS should be used as the correction to the non-
 dynamic electron correlation (through the the right geometry and
 wavefunction). What happens then when CISD tells you that an orbital
 energetically important though not needed in view of the reaction,
 description of the radical etc. it would be some parts of the
 dynamic correlation effect then?!
 2,  CASSCF in Gaussian 92 for me (and I guess for several others)
 stopped with the error message: rotation larger than 45 degree... If
 I understand well either a smaller step in wavefunction optimization
 would have been desirable or the active space was not well chosen and
 a non-selected orbital was crucial in the active space. Yet, I haven't
 got error message like this with the G94 routine, only warnings that
 large rotations are scaled and something like: I J=     12      26. I
 don't really understand the meaning of these numbers (i,if these are
 MO numberings, I have to say that eg MO- 12 was not selected to be in
 the active space, ii, in MOLCAS for example the user can define only
 the number and symmetry of the active orbitals, the exact MO not, how
 will then the routine pick the right orbital? iii, is it allowed to
 change orbitals of the active space during SCF).
 3, In case one loses the chk file and would like to keep it for
 further use, is the following way is correct to create the same chk
 file? 1, Calculate the HF-MOs at the starting geometry once again
 and picking the same MOs 2, converging the CAS wavefunction as it
 was done in the previous calculation and 3, reading in the
 wavefunction (to have the same active space) and having the
 converged CAS-geometry (not to redo the whole previous calculation).
 If it is correct can anyone tell me why I did not reproduce the
 same stationary point, but locating an other one (higher in
 energy). (As I know this method is used when one tries to find the
 right active space in a supermolecule by isolating it to fragments).
 If You had the patience and read my letter through, and You can help
 me in using CAS smoothly: THANK YOU!
 Varnai Peter
 PhD student
 varnai %-% at %-% ch.bme.hu