Subject: Bond breaking predicted by semiempirical methods

From: <[email-masked]>
Date: Thu, 10 Mar 1994 17:44:04 -0300

Dear Netters,
	Sometimes when I try to fully optimize the geometry of an organic
molecule using AM1, PM3 or any other semiempirical method, a situation arises 
where a particular bond distance in the molecule is optimized to a very
large value. 
	At first, one would be inclined to say that a case of structural 
instability is at hand and that such a molecule could not possibly exist.
	However, when I perform the same calculation using almost any
ab-initio technique, a very stable and nice structure is predicted.
	That happens even with molecules that have been synthesized,
are well known and whose x-ray structures have been determined. 
	I would like to know if any of you can send me some references 
on that. I will summarize the results to the net for the benefit of all.

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