Subject: TS refinement

From: <[email-masked]>
Date: Fri, 03 Nov 1995 11:25 -0800 (PST)

To the lot:

Here are two very helpful responses regarding refinement of TS 
structures in MOPAC; so that the IRC will work properly.

tim meehan

---------------------
The "gradients too large" _I think_ probably arises because somewhere along
the line of the IRC calculation, there is a FORCE calculation.  FORCE
expects to see the the GNORM<5.0.  You can _probably_ overrride this with a
LET
keyword. . .But I wouldn't advise this.  Try an EF to refine the structure
(I guess you should use TS since this is t he transiition state),
specifying a GNORM on the order of 2 or so.  Do  a FORCE to be sure you are
still at the TS (one -ve root to the Hessian), before trying the IRC=+/-1
stuff.  If your are still in the ballpark, you should be able to reduce the
gradient even further. . .this is physically "real" since the residual
forces on the atoms will influence the IRC. . .or so it seems to have
happened to me!

>From Painful experience, there are not shortcuts.  If you have more than
one -ve frequency, use the prcedure Stewart outlines to "escape from a
hilltop".

Hope this is helpful,
sb

-----------------------
I've recently found that a useful trick for refinement of stubborn transition
states is to take the best that you have & use the SIGMA keyword in *AMPAC*
[MOPAC hasn't got it].  It sometimes works when nothing else will.

A word of warning:  I've found instances of when AM1 **seems** to be closing
in on a ts, but there's NO WAY to refine it-- if I tried, the molecule fell
apart.  When I ran the corresponding reaction coordinate with ab initio me-
thods, I found the reaction to be monotonically decreasing in energy toward
fragmentation.

Of course, I don't know what you've tried, or what your ridiculous TS's look
like, but I hope these experiences may be of some use.

Good luck!
Irene Newhouse