Subject: Re: CCL:CCL: G92: SCF Convergence Problem: 6-31+G*

From: Dale Andrew Braden <[email-masked]>
Date: Sat, 5 Jul 1997 17:37:30 -0700 (PDT)

Dear Ben,

Maybe level-shifting using SCF(VShift=10000) will work, but you'll want to
drop that value to zero eventually.  Every time I have SCF convergence
problems, it has always been due to the wrong orbitals being occupied by
the initial guess.  But this results in a failure at the first step, and I
notice that your calc fails at the second step.  Still, it might be that
there are orbitals that are very close in energy, and which ones get
occupied depends sensitively on the geometry.  So the wfn is fine for the
initial geometry, but after the nuclei get moved a little, this wfn
becomes inappropriate.  In this case, you'll have to modify it using
Guess(Read,Alter).  I never know which orbitals to switch, but so far it
has always been the HOMO or HOMO-1 with the LUMO or LUMO+1.

You can have a look at David Young's website, where he has helpful
information on SCF convergence problems:

http://www.auburn.edu/~youngd2/topics/

Good luck, 

Dale Braden
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253
[email-masked]