Subject: G98: TS optimization

From: "Dale A. Braden" <[email-masked]>
Date: Wed, 17 Nov 1999 09:01:41 -0800 (PST)

Dear CCL,

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?

Best wishes to all,

Dale

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