Subject: Re: CCL:G:geometry optimization methods

From: "Dr. Heinz Schiffer" <[email-masked]>
Date: Tue, 08 Apr 1997 09:25:23 +0200

Dale Andrew Braden wrote:
> 
> Dear CCL,
> 
> A brief question:  Are there any circumstances in which a geometry
> optimization in Cartesian coordinates would be *faster* than in redundant
> internal coordinates?
> 
> The reason I am asking is because I am trying to optimize the geometry of
> a hydrogen-bonded dimer in a solvent medium using Gaussian 94, and the
> change in the two intermolecular Z-matrix coordinates (distance and angle)
> remains large for many, many iterations.  The geometry isn't exactly
> oscillating between two structures, but it isn't settling down, either,
> and I thought that perhaps by changing the optimization method from
> redundant internal coordinates (the default method in G94) to cartesians,
> the optimization might proceed more smoothly to a converged minimum.  I
> don't want to try this unless it can work, because the calculation is
> expensive.

Hi Dale,

Optimization with Z-matrix coordinates is always the worst that you
can do, next come cartesian coordinates, and the best are the natural
internal coordinates of Pulay. Cartesian coordinates are o.k. if you
have a good initial guess of the Hessian matrix (e.g. computed by
a less expensive method). See : Jon Baker, Techniques for Geometry
Optimization : A Comparison of Cartesian and Natural Internal
Coordinates, J. Comput. Chem. 14(9) (1993) 1085-1100.

Ciao,
Heinz

-- 
Dr. Heinz Schiffer		Phone   ++49-69-305-2330
Hoechst CR&T			Fax     ++49-69-305-81162
Scientific Computing, G864	Email   [email-masked]
65926 Frankfurt am Main		        [email-masked]

---

From: John McKelvey <[email-masked]>
Date: Tue, 08 Apr 1997 11:16:02 +0100

Dr. Heinz Schiffer wrote:
> -------This is added Automatically by the Software--------
> -- Original Sender Envelope Address: [email-masked]
> -- Original Sender From: Address: [email-masked]
> [email-masked]: Everybody | [email-masked]: Coordinator
> [email-masked]: HELP CHEMISTRY or HELP SEARCH | Gopher: www.ccl.net 73
> Anon. ftp: www.ccl.net   | [email-masked] -- archive search
>              Web: http://www.ccl.net/chemistry.html

I think I do not agree entirely with this.  Localised sub units, such as
rings whose relative torsion angles are uncertain pose challenging
problems. A bad initial guess at the angle will make cartesians a bad
choice, and a good Hessian won't help much.  Then, either Z-matrices or
redundant internals are FAR superior.  If the torsion has a low barrier
then the Hessian becomes very important.  Torsion angles and all their
characteristics are rampant in dyes, and we have to worry more about
then than any other single geometry factor.

Regards,

John