Subject: G94:scf=qc/diis problem

From: "Dale A. Braden" <[email-masked]>
Date: Wed, 15 Apr 1998 07:40:48 -0700 (PDT)

Dear CCL,

To find a converged DFT wave function for an open-shell anion in a
dielectric medium, I had to use the quadratically convergent SCF method.
It worked.  However, the Gaussian 94 manual seems to suggest that this
method is among those that tend to converge to the nearest solution, which
might not be a true minimum with respect to orbital rotations.  So, once I
had the converged SCF=QC solution (which was obtained using SCF=TIGHT), I
used it as the initial guess for another single point calculation using
the default DIIS method.  I could not get this to converge, even by
relaxing the convergence criterion, using extra integral accuracy (i.e.,
using SCF=novaracc but not SCF=tight) or level-shifting.  The QC wave
function was found to be stable (stable=opt scf=qc). 

I am simply curious to know why the DIIS method fails so badly when given
an already optimized wave function.  What property of the system (density
matrix? orbital energies?) causes DIIS to be so sensitive that it never
converges?

Best wishes to all,

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