Subject: MOPAC calculation on radicals - summary

From: [email-masked]
Date: Thu, 1 Apr 1999 15:42:57 +0100

In reply to my question:

> I'm interested in trying to model free radical alkene polymerisation reactions
> using MOPAC (AM1, PM3).  There appear to be two possibilities for calculations
> on radicals - the Dewar half-electron method (which is invoked by the keyword
> DOUBLET) and UHF.   The results differ by a few kcal/mol.
>
> Can anyone advise me which option is likely to give the best results for
> reaction thermodynamics and activation energies of a radical adding to a
> double bond?  I'd also be interested in any comments on bond dissciation
> energies estimated by these methods.  I'll summarise any replies.

I received one answer - thanks to Andy Holder at Semichem ([email-masked]):

"The third alternative is the best for your situation:  configuration
interaction (CI)>  The reason is that both UHF and DHE suffer from serious
inadequacies.  The DHE method simply isn't correct theoretically, although
it works marginally for doublets.  In the case of UHF in the current
semiempirical scheme, the results are not "spin pure" and have higher
multiplicities mixed in.  (You can tell this by looking at the S**2 value,
which is usually significantly different from the expected value.)  If the
two points you are trying to compare are not the same amount of "spin
impure" they are are not strictly comparable energetically.

"In contrast, CI results are "spin pure" and they also allow orbtal switching
during the course of a calculation on a reaction, as well as change in spin
state.

"MOPAC does have a CI module, but it is pretty limited and painfully slow.  I
suggest that AMPAC is a far better choice.  One of our collaborators is a CI
expert and our CI module is the fastest, most complete, and easiest to use of
anyone's."

Semichem will supply a demo of AMPAC on request.  Subsequent CI calculations
have
shown similar results to the half-electron heats of formation for my radicals.

Steve Rogers
([email-masked])