Subject: G98 CASSCF Efficiency Considerations

From: Ioannis Kerkines <[email-masked]>
Date: Wed, 3 Apr 2002 18:20:00 +0300 (EET DST)

Dear listmembers,

I am trying to perform with Gaussian 98 a CASSCF calulation with 4
electrons in 4 orbitals for a diatomic molecule using a basis set of 224
functions. I have encountered many problems so far regarding efficiency
and I would like to ask if anybody has any experience with this.

First of all, I am unable to do a "conventional" (SCF=Conven) calculation
because I have g functions in the basis set, whereas the subroutine
performing "conventional" calculations suppotrs only up to f functions.
The direct calculation of integrals needs much memory which is not
available. In fact I get a message:

  InCore calculation needs more space: 341070520
  In this calculation the integrals will be
 calculated in every iteration
  and Contracted with The Density Matrices

so the actual ("direct") calculation needs very much time to be done.
Furthermore, the CASSCF iterations after approaching convergence (very
very slowly), they start to diverge (very very slowly again), until
reaching the max. no of iterations (64 in this case) and then stop.

I tried to change the convergence options by imposing quadratic
convergence ( (CASSCF,4,4,QC,FullDiag) but no luck, I get an error message
at the beginning of the iterations) or Davidson Diagonalization (same
convergence behaviour as mentioned above).

I also use a very good guess (I have compared it with Molpro which takes
so much less time and converges easily).

Does anybody have any experience with this?

Best regards,
Ioannis