Subject: G94 & G98 DFT (Summary)

From: Emadeddin Tajkhorshid <[email-masked]>
Date: Mon, 19 Apr 1999 12:27:53 -0700

Dear all

Last week I sent a question regarding the (small) differences between
the G94 and G98 programs with respect to the SCF energies and geometry
optimization. Thanx to all who replied to my question. Here is the
original question and a summary of the replies I got. 
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Question:
---------
I have experienced something using geometry optimization in G94 and G98
that I would like to check with you. I started from a G94-optimized set
of  coordinates for glycine. Using a DFT method (B3LYP/6-31G**), on the
same platform (IBM/SP2) I started the same job in G94 and G98. These two
jobs, while having the same optimization criteria (Default values) and
the same starting geometries (and I suppose the same initial guess?)
give different results. In one case the geometry is optimized and in the
other case, the job does not meet the 'Maximum displacement' and 'RMS
displacement' criteria after the first cycle. Examination of the energy
also shows that we have different values after the first SCF. 

Although the differences in this case are not very large, I would like
to know why they happen. If I am not mistaken there are some
modifications with respect to the optimizer in G98, but I think in the
case of this example, the difference is happening at the level of
wavefunction. 

Did anybody else experience such differences at the wavefunction or
geometry level? If so, how large are they? 

---------------------------G94 job---------------------------------
First SCF:

 SCF Done:  E(RB+HF-LYP) =  -284.437033067     A.U. after   15 cycles
             Convg  =     .4529D-08             -V/T =  2.0089
             S**2   =    .0000
 KE= 2.819396660737D+02 PE=-1.026794554909D+03 EE= 2.802029338205D+02

:::::
         Item               Value     Threshold  Converged?
 Maximum Force             .000028      .000450     YES
 RMS     Force             .000009      .000300     YES
 Maximum Displacement      .000293      .001800     YES
 RMS     Displacement      .000108      .001200     YES
 Predicted change in Energy=-1.479922D-08

---------------------------G98 job ---------------------------
First SCF:

 SCF Done:  E(RB+HF-LYP) =  -284.437035418     A.U. after   15 cycles
             Convg  =     .4595D-08             -V/T =  2.0089
             S**2   =    .0000
 KE= 2.819396670445D+02 PE=-1.026794557138D+03 EE= 2.802029327275D+02

::::
 Maximum Force             .000028      .000450     YES
 RMS     Force             .000012      .000300     YES
 Maximum Displacement      .004390      .001800     NO 
 RMS     Displacement      .001377      .001200     NO 
 Predicted change in Energy=-7.549560D-08
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... answers:
------------
Dear Emad,

G94 and G98 DFT differ in at least two points.
1) The default weighing scheme for the numerical integration is now
Stratmann/Scuseria. It was different in G94 (the Becke scheme ?).
2) The SCF cycles are now started with a superposition of states instead
of the
pure guess. This _might_ lead to differences, but does not seem to be
the case in
your calcs.

Stefan
______________________________________________________________________
Dr. Stefan Fau
Fachbereich Chemie, AK Frenking
Philipps-Universität Marburg
35032 Marburg, Germany
[email-masked]
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   Emad,

   There is a minor difference in energies, most likely due to the
change in default numerical integration, G94 used Becke while G98 uses
Strattman/Scuseria points and weights. The minor energy difference is
consistent with this.  Also the gradient summaries are identical which
is
consistent with having the wavefunction the same.

   The obvious difference is in the predicted displacment.   G94 and
G98 use different sets of redundant internals and start from an
empirical
valence hamiltonian.  The variations here are likely what is causing the
difference.  I suspect that if you used Opt=CalcFC you would have much
closer displacement predictions.
-- 

  Douglas J. Fox
  Director of Technical Support
  [email-masked]
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Hi,

if I am not mistaken (which might well be the case) then G94 and G98 use
different levels of integration grids for DFT calculations. This would
perhaps explain the differences that you see.

Please summarize!

Best regards, Georg

--
==============================================================
Dr. Georg Schreckenbach           Tel:     (USA)-505-667 7605
Theoretical Chemistry T-12        FAX:     (USA)-505-665 3909
M.S. B268, Los Alamos National      E-mail:  [email-masked]
Laboratory, Los Alamos, New Mexico, 87545, USA
Internet:    http://www.t12.lanl.gov/home/schrecke/  ***new location!***
==============================================================
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Thanx again.
-- 
Emad
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E. Tajkhorshid, Ph.D. 
German Cancer Research Center; DKFZ             Tel: +49 6221 42 2340
Dept. Molecular Biophysics (H0200)              FAX: +49 6221 42 2333
P.O.Box 101949            http://genome.dkfz-heidelberg.de/users/emad
69009 Heidelberg, Germany     Email: [email-masked]
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