Subject: RE: negative vibration frequency

From: "Shobe, Dave" <[email-masked]>
Date: Mon, 23 Jul 2001 11:37:46 -0400

For Gaussian at least, you might be able to create a nonstandard route that
does this, using the IRC code.  This is an advanced topic I can't help you
with, and in most cases will waste more CPU time than the "manual"
altermative I will describe shortly.  (The reason is that you'd be
reoptimizing from a point that's near the TS, far "uphill" from the
minimum.)
 
A better strategy is to use a visualization program that animates
vibrations.  This gives you a "movie" to stare at and see what is happening
& how the atoms are moving.  Very often it's a simple methyl group rotation
or the like.  Then, make a guess where the minimum is for that
imaginary-frequency vibration, and optimize from there.   If you can't make
a reasonable guess, *then* you might want to use IRC, which follows the
imaginary frequency path downhill.
 
Isn't there some quotation about "brain time" saving CPU time?  I wonder how
long that will be true, given Moore's law? (Perhaps the quotation is already
out of date...)
 
--David Shobe 
Süd-Chemie Inc. 
phone (502) 634-7409 
fax     (502) 634-7724 
email  [email-masked] 

Don't bother flaming me: I'm behind a firewall. 

-----Original Message-----
From: Hui-Hsu (Gavin) Tsai [mailto:[email-masked]]
Sent: Thursday, July 19, 2001 11:31 AM
To: [email-masked]
Subject: CCL:negative vibration frequency

Hi CCLers:
 
    I have a question about the negative frequency in the frequency
calculation.
For larger or complicate molecule, the geometry optimization usually can not
reach the minimum point, but the saddle point with some negative and small
vibration frequencies. To adapt and try to reoptimize the molecule wastes
CPU.
Is there any geometry optimization algorithm which can follow the negative
frequency and reach the minimum point automatically?
any comment is welcome! Thanks!
 
Gavin