Subject: Summary: scf = qc/diis problem

From: "Dale A. Braden" <[email-masked]>
Date: Fri, 17 Apr 1998 08:06:40 -0700 (PDT)

Dear CCL,

My original question was whether anyone knew why the DIIS method should
fail to converge a wave function which had already been optimized with the
quadratically convergent method.  This makes no sense to me, and it is
still a mystery.

There were several suggestions as to why DIIS might have trouble.  The
Gaussian 94 manual indicates that the QC solution might not be the global
minimum, so I did a wfn stability check, and found that the solution was
indeed stable (at least under the orbital rotations that were made).  The
smallest eigenvalue of the stability matrix was 0.07 -- does this suggest
a shallow surface?

The HOMO-LUMO gap was nearly 2 eV, so it's not surprising that
level-shifting did not help the DIIS calculation to converge. 

Finally, the value of spin contamination, S^2, for the QC solution was
only 0.7613 (for a doublet), which is excellent for the B3LYP density
functional I was using.

Up to this point, whenever I have had trouble with DIIS convergence, the
problem has always been in the initial guess, i.e., the wrong orbitals
were occupied.  When the guess is correct, and DIIS fails, then I try
increasing the integral accuracy and relaxing the convergence criterion
(i.e., not using scf=tight).  When this fails, I switch to QC, which takes
much, much longer, and only succeeds about half the time.  One respondent
mentioned that SCF=DM is even more certain to find a solution, but that it
takes even longer than QC. 

What surprised me about this particular problem was that DIIS was crashing
even when it was given a true solution to start with-- no changes were 
necessary.  A solution is a solution, after all!  Maybe I should just
check Pulay's original article on DIIS.....

Best wishes to all,

Dale

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