Subject: finding transition states using MOPAC6.0

From: [email-masked] (Max Muir )
Date: Sun, 29 Aug 93 15:05:42 -0700

Dear CyberChemists,

 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. I have one and one only
 negative root.  What's the problem?  What's the relationship
 between the GRAD reported in the line beginning 'CYCLE' and the
 'GRADIENT NORM' resulting from the FORCE calculation.  What do I
 need to do to improve my GRADIENT NORM?  How good does it have to
 be for publication?

 tx,

 Max