Subject: CASSCF questions

From: "Varnai Peter" <[email-masked]>
Date: Fri, 3 Nov 1995 15:50:17 GMT+100

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
[email-masked]