Subject: RE: IRC on G03

From: "Philip Hultin" <hultin/at/cc.umanitoba.ca>
Date: Thu, 9 Dec 2004 13:29:41 -0600

Alessandro:

We did a fair bit of IRC a while ago (look for upcoming papers in J Phys
Chem A) and we found that sometimes a calculation would begin to have
convergence failures as it moved further from the initial structure (i.e.
the TS).  This was because the force constants became less reliable the
further one got along the IRC.

In such cases, if we wanted to move further along the IRC, we needed to
specify CALCALL to get fresh force constants at each point.  This is a bit
time-consuming but not nearly as slow as a failed calculation.  Using small
step sizes in the IRC also helped a bit.

My understanding of the documentation is that CALCFC only does the force
constant calculation at the initial structure, NOT at the beginning of each
point along the IRC - that is what CALCALL does in this case.

After doing this a few times, we concluded that what Gaussian should
consider building into the IRC was an option to specify when new force
constants should be calculated.  It would be nice, for example, to be able
to calculate force constants at every Nth point along the IRC - it would
keep the calculation on track but take much less time than calculating them
at each new point.  Another useful variation would be to move N points along
the IRC using extrapolated force constants and only then start doing
calculated force constants.  After all, in most cases one would expect force
constants to be drastically different near the "starting material" and
"product" states than they are near the TS, but that the differences would
initially be small.  Any thoughts about this suggestion would be welcome.

P. Hultin

Associate Professor of Chemistry

University of Manitoba

---

From: Steve Williams <willsd/at/appstate.edu>
Date: Thu, 09 Dec 2004 15:23:04 -0500

At 01:29 PM 12/9/2004 -0600, Philip Hultin wrote:

>time-consuming but not nearly as slow as a failed calculation.  Using 
>small step sizes in the IRC also helped a bit.
>
>
>
>constant calculation at the initial structure, NOT at the beginning of 
>each point along the IRC that is what CALCALL does in this case.
>
>
>
>to calculate force constants at every Nth point along the IRC it would 
>keep the calculation on track but take much less time than calculating 
>them at each new point.  Another useful variation would be to move N 
>points along the IRC using extrapolated force constants and only then 
>start doing calculated force constants.  After all, in most cases one 
>would expect force constants to be drastically different near the starting 
>material and product states than they are near the TS, but that the 
>differences would initially be small.  Any thoughts about this suggestion 
>would be welcome.

I've long thought that a calcsometimes option would be useful for 
optimizations as well, particularly for transition state 
optimizations.  I've done this manually for some tricky transition state 
optimizations by limiting the number of steps, computing new force 
constants, doing several more steps, etc. Automatically computing the 
Hessian every 5 or 10 steps could be useful in several types of 
calculations.  Count me on supporting this as a future Gaussian option. 
(Mike, Doug, did you hear this?)
Steve Williams
Department of Chemistry
Appalachian State University
Boone, NC 28608