Subject: ab initio convergence , parameters

From: Wentian Liu <[email-masked]>
Date: Mon, 12 May 1997 20:29:31 -0400

Hi,
    When I used ab initio UHF to optimize geometry of a vanadium(IV) 
complex with about 65 atoms,  it always failed to converge. Could anyone 
tell me something about the convergence problem of ab initio. What 
factors might cause convergence-failing? Is there any way to avoid it? Is 
ab initio appropriate to geometry optimization of transition metal complexes?

   In addition, is there anyone knowing where I can get parameters of 
vanadium for semi-empirical calculations such as MNDO, MNDO/d and AM1?

Any suggestion or help is greatly appreciated.

Wentian Liu
North Carolina State University

---

From: <[email-masked]>
Date: Mon, 12 May 1997 22:57:19 -0400 (EDT)

There are two main factors affecting geometry optimization:
potential energy surface and force constant matrix(Hessian) used
in optimization. If a molecule has a very flat potential, i.e. the
energy being insensitive to geometry change, the optimization often
has trouble. By default, optimization uses a guessed hessian matrix,
which is updated in each optimization cycle. However, the updation often
goes wrong. You may kill the job and resubmit it with the best
geometry. But DO NOT READ OR USED OLD FORCE CONSTANTS!  If this still
does not work, you have to use analytical hessian in each cycle when
possible. Gaussian has a option to do that (OPT=CalcAll).
Hope this will help.
=================================================================
Dr. Buyong Ma             [email-masked]
Computational Center for Molecular Structure and Design
Department of Chemistry
University of Georgia
Athens, Georgia 30602 USA            Voice (706) 542-2044
=================================================================

On Mon, 12 May 1997, Wentian Liu wrote:

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