Subject: G94(Sum.+Q): Problem with PES Scan in DFT

From: "M. Nicklaus" <[email-masked]>
Date: Fri, 31 Jan 1997 17:59:33 -0500 (EST)

Dear CCL'ers,

This is the, maybe preliminary, summary of the responses I got 
to my question concerning a problem with PES Scan in DFT in G94.
A few follow-up questions are appended.

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Question:
=========

? I seem to be having a problem with a Potential Energy Surface (PES) scan
? in G94.  I am running a relaxed PES scan on a nucleoside analog, trying
? to rotate the base.  I am running this job at the DFT method/basis set
? B3LYP/6-31G(d).  I'm using cartesian coordinates (a newzmat-generated
? input file with a Z-matrix didn't work previously), pre-optimized at
? the same level of theory from X-ray coordinates.
?
? For the first few conformations, everything seems to have proceeded nicely;
? i.e., the energy started out some 20 kcal/mol or so above the (presumed)
? global energy minimum, and then came down, during the partial optimization,
? to a reasonable value a few kcal/mol above the global min.
? With the current conformation, however, the job seems to be stuck, since
? for nearly 20 steps now, the energy has only been oscillating in a narrow
? band. [rest omitted]

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Responses:
==========

From: Jan Labanowski <[email-masked]>

Try to see, if you have very closely spaced HOMO/LUMO -- usually, due
to Self Interaction problem of DFT, the SCF will oscillate in this case.
I do not know how to fix it in G9X, since I do not use it for DFT, but
other problems have some options, like DMIX (a portion of new density
to be mixed with old density for new SCF iteration), or freezing occupances,
or level shifting technique, or smear approach. If some of those are
available in gaussian, use them. Going on with the run will not help,
it will just keep oscillating. Of course, I may be wrong as to the cause,
since I did not see your MO energies.

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From: [email-masked] (Jennie Weston)

The problem is probably that the optimizer routine is taking too big
steps for your pot E surface. You are apparently in the region of a very
flat minimum (maybe the thing is quite rotationally flexible??). The routine
must step smaller in order to find the stationary point. Kill the job at 
once, further comp time is just wasted. Cut out the geometry corresponding 
to the lowest energy in the many optimization cycles. Use it to make a new
input file (so you don't loose the work already done) (so you don't loose
 the work already been done on the optimization). Include (For Gaussian)
the option opt=tight in the key word line. If this still doesn't work, use
opt=verytight. This decreases the tolerances for the optimization routine
and forces it to step smaller. However, it takes more comp time.

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From: [email-masked] (John Nicholas)

Marc, yes I have seen it often with dft in G94. I do not know the origin
of the problem. I would guess the job may never converge. However,
you should check the forces, if the forces are still high, then maybe
something is wrong other than g94's problem with convergence.

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From: [email-masked]

  Is it possible that you entered the step in degrees while the program read
it as radians?  Sometimes the documentation is incomplete or just plain
wrong.

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Follow-up questions:
====================

1. Taking up Jennie's suggestion, I realized that this conformation
   may indeed be in a 'difficult' region of the energy surface
   It is quite possible that it is very close to a local maximum
   relative to the scanned torsion, i.e. close to the top of the 
   rotation barrier.  With this---and only this---torsion constrained
   to the presribed value in the scan, I think it is very well possible
   that the energy surface with respect to the remaining coordinates
   is very flat, or otherwise 'weird looking'.  Any thoughts, or
   experiences on this point?

2. Can one restart a PES scan job with tighter convergence for the
   optimization by using Opt=(Restart,Tight,AddRedundant); or will 
   restarting the job from the checkpoint file ignore the new 
   keyword "Tight" and proceed with the same settings as before?

3. How do I extract a specific structure from a PES scan checkpoint
   file; i.e., how do I tell newzmat that I want, say, the (structure
   of the) 17th step in the optimization of the 3rd conformation?

Thanks to all who responded, and thanks in advance to future responders.

Marc

------------------------------------------------------------------------
 Marc C. Nicklaus                        Lab. of Medicinal Chemistry
 e-mail: [email-masked]               National Cancer Institute, NIH
 Phone:  (301) 402-3111                  Bldg 37, Rm 5B29
 Fax:    (301) 496-5839                  BETHESDA, MD 20892-4255    USA
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