Subject: Heat of formation and stationary points

From: Deepak Singh <[email-masked]>
Date: Thu, 01 Jul 1999 14:28:17 -0400

Hello again,

I have been running MOPAC calculations on systems with 2 or more
molecules (not bonded to each other).  I have noticed something
interesting.  When I do not constrain anything, the SCF calculation goes
along fine.  However when I constrain 2 or more  atoms, I almost always
have my Heats of Formation eventually becoming self consistent, but not
my gradiets and the arc file reports that although the SCF has been
achieved, the geometry is not at a stationary point.  What does this
mean, especially in a physical sense?

Regards

Deepak.

--
**********************************************************************
Deepak Singh                        Tel : (315)443 1739 (w)
Graduate Student                          (315)472 9659 (h)
Dept. of Chemistry                Fax : (315)443 4070
Syracuse University               email : [email-masked]
1-014 CST, Syracuse               URL : http://web.syr.edu/~desingh
NY 13244

"Violence is the last refuge of the incompetent." --- Salvor Hardin
**********************************************************************

---

From: Mauricio L Cafiero <[email-masked]>
Date: Thu, 1 Jul 1999 11:40:56 -0700 (MST)

Hello:
	A geometry optimization finds a minimum based on the gradients
with respect to atomic coordinates. When you constrain any atoms, the
optimization cannnot find a true minimum, and hence your total gradient
will not become very small. 
	So your gradients do not converge because you do not reach the
minimum and you do not reach the minimum because you have constrained
atoms. The SCF will work at any geometry, so you can find an energy at
that non-optimum geometry.

========================================================================
Mauricio Cafiero
University of Arizona
Department of Chemistry 

On Thu, 1 Jul 1999, Deepak Singh wrote:

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