Subject: Re: CCL:SCF Convergence problem in G03

From: john <jmmckel<<at>>attglobal.net>
Date: Wed, 04 Aug 2004 20:02:59 +0100

There may be several issues/approaches, IMHO

First, look at the initial gues around the homo-lumo gap..  Are there
unfilled orbitals that are essentially degenerate with orbitals to be
filled?  If so a single determinate solution may not work.... and bad
convergence would not be a surprise

Second, if the above is not the case, then start off using a lower level
of theory and/or basis sets, and see if that will converge...  SCF
convergence can be more art than science..

Roothan did not guarantee us SCF convergence, only the right to try.

:-)  Regards,

John McKelvey

Tapas Kar wrote:

> Hi experts,
>
> I am getting SCF convergence problem with single-point calculations on
> large C200-O3 to C260-O3 systems using B3LYP/6-31G* basis in G03. I
> tried vshift=300, SCF=QC  and different guess with no luck. Also tried
> anion(-2) and cation (+2)of the same.  Energy is highly oscillating >
> from one step to other, even +/- 20au. Is there any option in G03 to
> get my job done. Gamess looks don t allow more than certain number of
> basis functions.
>
> Thanks
>
> Tapas
>
>
> -------------------------------------------------------------------------------
>
> "We owe a lot to the Indians, who taught us how to count, without
> which no worthwhile scientific discovery could have been made."
>                                            - Albert Einstein -
>
> -------------------------------------------------------------------------------
>
> Tapas Kar, Ph. D
> Department of Chemistry & Biochemistry
> UtahStateUniversity
> Logan, UT84322-0300
> Tel: 435-797-7230
>
> Fax: 435-797-3390
> Email: tapaskar<<at>>cc.usu.edu
> Web: http://www.chem>.usu.edu/faculty/Tapas/index.html
>          http://www.chem.usu>.edu/~tapaskar/index.html
>