Subject: Responses to 'finding transition states with MOPAC6.0'

From: [email-masked] (Max Muir )
Date: Mon, 30 Aug 93 21:15:12 -0700

Dear CyberChemists,

thanks for your help.

 >using MOPAC6.0 I have been attempting to correlate calculated
 >heats of formation of some transition states with experimental
 >activation energies for the following reaction
 >
 >              H       R1                            R1
 >     .         \     /                            ./
 >  tBu     +     C = C      ------->   tBu - CH2 - C
 >               /     \                             \
 >              H       R2                            R2
 >
 > I am getting some credible looking structures for the transition
 > states.  However, I cannot seem to get close to the saddle point, 
 > where the gradient normal is zero.  I set SCFCRT=10^-12 and
 > GNORM=0.0001, and the calculation exits normally thus-
 >
 > CYCLE:   9 TIME: 195.67 TIME LEFT: 356495.2 GRAD.:     0.000 HEAT: 4.288334
 >
 > But when I do a FORCE calcn to verify the single -ve ROOT, MOPAC reports that
 > GRADIENT NORM = 0.12229, which is not really good enough for publication is 
 > it?  I have one and one only negative root.  What's the problem?  What's the

 (Hysterical questions deleted).

 I fixed up WRITE.f to write out gradient norm as the calculation finishes
 (like AMPAC).  Thus . . 

 CYCLE:   6 TIME: 240.34 TIME LEFT: 357099.6 GRAD.:     0.000 HEAT: 4.288334    
          HESSIAN CALCULATED NUMERICALLY
 CYCLE:   7 TIME: 240.05 TIME LEFT: 356618.8 GRAD.:     0.000 HEAT: 4.288334    

 -------------------------------------------------------------------------------
 GRAPH UHF AM1 NODIIS ITRY=500 MMOK DMAX=0.20 TS SCFCRT=0.000000000001 +
 GNORM=0.0002 LET NOINTER RECALC=1
 INSIGHT generated AMPAC/MOPAC input file
 DATE: Tue Aug 24 18:44:09 1993

     GEOMETRY OPTIMISED USING EIGENVECTOR FOLLOWING (EF).     
     SCF FIELD WAS ACHIEVED                                   

                                AM1    CALCULATION
                                                       VERSION  6.00
                                                       30-Aug-93               

          FINAL HEAT OF FORMATION =          4.28833 KCAL

          TOTAL ENERGY            =      -1257.77074 EV
          ELECTRONIC ENERGY       =      -6795.64588 EV
          CORE-CORE REPULSION     =       5537.87514 EV

          mxm GRADIENT NORM           =        0.0001815  <-------<<  Yippee!!
          IONIZATION POTENTIAL    =          7.96377
          NO. OF ALPHA ELECTRONS  =         25
          NO. OF BETA  ELECTRONS  =         24
          MOLECULAR WEIGHT        =    113.222

          SCF CALCULATIONS  =              491
          COMPUTATION TIME = 28 MINUTES AND  15.740 SECONDS

 However, it looks like there is no easy way to get the refined structure
 into MOPAC for the force calcn without altering the file readers/writers.

 And in any case it does not seem to matter.  

 I here append the responses.

_______________________________________________________________________

Date: 30 Aug 93  8:08 +0200
From: Frank Jensen <[email-masked]>
Subject: mopac ts's

	Max,
	Your "problem" is simply that the geometry as printed in the
output or achieve file only contain some 5-6 digits. In the optimization
the full accuracy of the geometry is produced and you may obtain very
small gnorm's, but the final geometry is only printed with an accuracy
that gives you the gnorm of ~0.1 in the force calculation. A force
calculation with a gnorm < 1.0 is fine. If you realy need the full accuracy
you would have to change the format statement in the code to print out
more digits.
	Frank
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From: [email-masked] (Jack Smith)
Subject: Re: finding transition states using MOPAC6.0

Max:

   If you're using SADDLE to locate the transition state, then follow it
with a gradient minimization (I usually use EF) before doing the FORCE
calculation.  SADDLE does not appear to refine its final geometry.  I'd
save the trial saddle point before doing the gradient minimizatin, though -
I have had many "SADDLE" points just fall off to one side of the saddle
point and have to start all over.  I'd also follow the FORCE calculation
with IRC calculations along both directions of the negative root to confirm
that the transition state indeed connects the desired reactants to desired
products.

   According to the MOPAC documentation, the PRECISE keyword should
generate publication quality results (as far as SCF convergence and
Gradient Norms are concerned).
 
_______________________________________________________________________

From: [email-masked]
Subject: Re: finding transition states using MOPAC6.0

I've had problems with a second step in MOPAC 6 'claiming' a larger
gradient norm than the previous step did.  I've not gotten a satis-
factory answer to this question.  In your case, you need to re-
member that nothing is really 0 with floating point arithmetic on
a computer--it's all roundoff error.   I'd be quite satisfied with
a gradient norm of about 0.1 for publication, & I suspect most 
people would, especially for a transition state..
Irene Newhouse
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