Subject: RE: CCL:IRC with GAUSSIAN92

From: [email-masked] (Ron Shepard)
Date: Wed, 28 Sep 94 12:31:44 CDT

[email-masked] (Antonio F. Ramos) wrote:

>Dear netters:
>	I've been trying IRC calculations with G92. There is no problem 
>from TS to reactives or products, but I'd like to be able to follow the
>lowest frequency normal mode from reactives or products to the opposite
>side where the TS is.

Although this was originally a G92 question, there are a couple of points
that might be of general interest to the maillist.  First, the IRC is
everywhere parallel to the gradient (in mass-weighted cartesians, etc.,
you know what I mean :-).  However, there are an infinite number of paths
that are parallel to the gradient that lead to a minimum on a surface,
all of which (ignoring spatial symmetry and other special situations),
are parallel to the lowest frequency normal mode at the minimum point,
and only one of which happens to pass through the TS.  Therefore, it is
relatively difficult to try to construct the IRC in reverse, that is,
from a minimum to a TS.  It is probably more efficient to search for the
TS using some stable method, then determine the reaction path from the TS
to the connecting minima in the downhill direction, although this is an
active research topic.

Secondly, the gradient is in general parallel to a normal mode only at a
stationary point, not everywhere along the IRC path.  The normal mode to
which the gradient is parallel at one stationary point (e.g. the minimum)
is not necessarily the same normal mode at a different stationary point
(e.g.  the TS).  Therefore, it is not always possible to follow a normal
mode from a minimum to a stationary point.  Even if a normal mode does
connect the minimum to the TS, the path it takes is not necessarily
anything like a reaction path (IRC, least motion, or otherwise).
K. Ruedenberg and coworkers have recently published some nice examples of
how screwy these paths can be on some model surfaces.

$.02 -Ron Shepard