Subject: RO jobs on unsaturated organometallics

From: "Eric S. Ball" <[email-masked]>
Date: Sat, 4 Dec 1999 05:30:32 -0800 (PST)

Greetings,

I'm having great difficulty getting convergence for my
restricted open-shell DFT jobs on coordinatively unsaturated
iron compounds.  I'm using Jaguar for these (rob3lyp/lacvp**),
and I've used G94/98 in the past, so suggestions pertaining to
either package would help.  In Jaguar I've used vshifts of 0.2 -
1.0; accuracy settings (iacc) of quick, medium, and ultrafine;
As per hints in the manual I tried iacc=1 (ultrafine grids) with
the lac3vp** basis (largest available) and etot simply
oscillated.

If anyone has successfully dealt with a similar situation with
either Jaguar or G98, I'd love to start a little dialog. 
If/when I meet with success I'll edit down any conversations to
the pertinent points and repost them.

Thanks in advance,
Eric Ball

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From: "Dale A. Braden" <[email-masked]>
Date: Mon, 6 Dec 1999 07:28:47 -0800 (PST)

Dear Eric,

The best thing to do is email Schrodinger at least one of your input
files.  In my case, they ran the job themselves until they figured out how
to make it converge, and then emailed me the solution.  They're interested
in cases where their code fails.

Otherwise, I'd suggest trying a *smaller* basis rather than the largest
one, say lacvp.  Sometimes a large basis provides too many degrees of
freedom for a wfn that is a poor initial guess, but moving to a smaller
basis helps the converger find a solution.  Another choice is to use the
OCBSE SCF-converence method.  This worked for one of my problem cases.

Good luck,

Dale

Dale Braden
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253
[email-masked]
On Sat, 4 Dec 1999, Eric S. Ball wrote:

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From: "Richard P. Muller" <[email-masked]>
Date: Mon, 06 Dec 1999 08:50:06 -0800

> the pertinent points and repost them.

I can't speak for Gaussian, but this is a well-known problem in Jaguar
for DFT with transition metals. Schrodinger is working on a solution,
and I sent them some of our group's most catastrophic convergence cases
to test it on. But, as I understand the situation, this fix won't be
available for a while, so there are some things you can try.

People in our group here (Caltech MSC) find setting iconv=4,
dconv=1.0d-5, vshift=0.5, iacc=1 is a good workaround for these states,
although this by no means makes the problem go away.

GVB-DIIS convergence, the iconv=4 flag above, uses a slightly different
convergence scheme, which may work better. I don't know why DFT
convergence is so much harder than HF. I wrote the gvbdiis scheme as
part of my graduate thesis, and if I had known how much harder DFT wave
functions were, I would have focused on those wave functions instead of
the HF and GVB wave functions.

Another thing I try is to use a HF wave function as the initial guess.
Again, this doesn't always work, which is something that I find amazing.
I realize that the DFT and HF wave functions are different, but it
surprises me that they are so different that the HF isn't always even a
good initial guess.

I think the reason that metals are so hard is that they have a large
number of virtual orbitals very close in energy, and as the wave
function converges the ordering of the orbitals changes, which throws
off the convergence.

I hope this is some help. I certainly empathize with your frustrations
with these cases.

Rick
-- 
Richard P. Muller, Ph.D. 
[email-masked] 
http://www.wag.caltech.edu/home/rpm

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From: Kristan Markey <[email-masked]>
Date: Tue, 7 Dec 1999 03:37:11 +0100 (MEST)

Greetings,
The convergence problem with transition metal clusters is something with
which I have also had to deal. First off, Gaussian uses a Hueckel guess
for its initial guess: not the best choice. Try converging a calculation
with a small basis set and using the checkpoint file as an initial guess.
Although this does not always work, especially when there are many
virtuals close in energy, it is a good start. 
Secondly, restricted open shell wavefunctions and DFT don't mix. Indeed,
when I was working on Ni clusters, I experienced the same Etot
oscillations. I am not sure of the fundamental problems, but the
wavefunction converges much better if it is unrestricted. The
strict spin treatment is less important for DFT anyways.

Cheers,
kristan 

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