Subject: Summary: G98 TS optimizations

From: "Dale A. Braden" <[email-masked]>
Date: Mon, 22 Nov 1999 07:36:23 -0800 (PST)

Dear CCL,

Two weeks ago I posted a request for help in getting Gaussian98 to do a
transition state optimization when two modes had negative hessian
eigenvalues (original question below).  Thank you to all respondents. 
There isn't really a proper solution, although there is a workaround. 
There seems to be no general mode-following algorithm in G98, because
opt=ef is limited to 50 active variables and opt(tvector=n) isn't
recognized, even though it's in the manual. 

The workaround works only when the mode with the lowest eigenvalue is the
one of interest, which is probably the case most often.  So one can just
use opt(ts,calcfc,noeigentest).  The "noeigentest" option prevents the
program from stopping if two or more negative eigenvalues are encountered.
This happens all the time when there are internal rotors.

A few people suggested using the QST2 or QST3 methods, but I think these
also follow only the mode with the lowest eigenvalue, so they won't work
when a different mode is of interest.  (I did actually give QST2 a try,
but it could not generate coordinates for the TS structure.) 

Best wishes to all,

Dale

Dale Braden
Department of Chemistry
University of Oregon
Eugene, OR 97403-1253
[email-masked]

---original post---

I am trying to use Gaussian98 to locate a transition state.  Using
opt=(calcfc,ts) results in two negative hessian eigenvalues, and the job
fails.  Thus I need a way to specify which mode should be followed. 
According to the manual, this can be done by specifying tvector=N where N
is the mode number.  In fact, this keyword is not even recognized.  An
alternative is to use opt=ef to select the eigenvalue following algorithm. 
Unfortunately, this is limited to 50 variables, which is too small for the
molecule I'm looking at.  This leaves opt=noeigentest, in which case I
assume that the mode with the lowest eigenvalue is followed, but what if
one wants the other one? G98 prints out one eigenvector, in internal
coordinates, which is required to have a negative eigenvalue, even when
there are two or more negative eigenvalues.  What is needed is the
eigenvectors for ALL negative eigenvalues, in cartesian coordinates, so
that they can be visualized (animated) and the wrong modes identified. 
Then, at least, the geometry could be manually displaced downhill along
the modes that are not to be followed. 

How does one handle this situation in G98?