Subject: 2nd order saddle point (summary & more problems..)

From: Jussi Eloranta <[email-masked]>
Date: Tue, 25 Jul 1995 10:09:33 +0300 (EET DST)

First of all thanks to everyone who answered my posting.

My original question was:

> Is there a way to optimize to the geometry which is a minimum with respect
> to all variables except two for which it should be a maximum
> (ie. saddle point of order of 2). These two variables are some known Z-matrix
> variables.
> 
> I've looked at saddle, ts, and ef params for opt but they don't seem to
> help with this.
> 

Most people suggested using opt=(saddle=2) but as far as I've understood
I would have to be very near the right solution (which I don't know very well
- I'm trying to do AM1 based rotation barrier calculations to understand
where the right solution should be...). Otherwise I don't know where
this will converge. Since with this there is no way to tell which vars should
be maximized for energy and which ones should be minimized for energy.

The eigenvalue following allows me to minimize with respect to all other 
variables but maximize one variable. This also has a limitation of 50 active
variables (I have about 60). There don't seem to be any direct way of adding
a second variable to the energy maximized vars.

I also mentioned:

> BTW would this be as simple as choosing "-Grad" direction for all variables
> but these two for which it would be "+Grad" ?

Oh well... this gives me information of the direction we should progress
but I don't see any straight forward way of doing the line search since
it isn't really a minimization... May be the optimization could be done
separately for minimization and maximization. I'll give this a quick
test by writing a simple gradient based optimization routine on top of g92
which will try to maximize the energy with respect to my torsion variables. 
During the g92 run these torsion variables are frozen and are only changed
by the grad optim. program.

Any comments?

Best regards,

Jussi Eloranta

ps.