Subject: Anomalous PM3 Hydrogen Interactions

From: [email-masked] (Craig W. Burkhart)
Date: Fri, 17 Dec 93 14:46:22 -0500

Dear netters,

We have a query regarding anomalously stable close contacts between
nonbonded hydrogen pairs when using PM3.

We would be interested in any comments regarding a problem we
have encountered when using the semi-empirical method PM3. The
simplest example of this problem is the formation of a "stable"
"dimer" of methane. The geometry minimizes to a straight line
between C-H ... H-C bonds in the dimer. This dimer possesses
an apparent stabilization energy of about 2kcal/mole, with an
H ... H separation distance of 1.7 Angstroms.

We are concerned, as this effect is showing up in a host of crowded
molecules--for example, t-butyl cyclohexane, or even in all-gauche
conformation of n-pentane! We are not aware of any published
discussion of this problem, but it is leading to what may appear
to be erroneous geometries and heats of formation in many cases.

Is this problem an inherent weakness of PM3? Are there satisfactory
workarounds? Any references to discussions of this problem?

Thanks in advance. Send all responses to [email-masked],
and we will summarize for the net...

Sincerely,

Eilert Ofstead and Craig Burkhart
Goodyear Research

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Craig W. Burkhart, Ph.D.                   Senior Research Chemist 
E-mail: [email-masked]             The Goodyear Tire & Rubber Co.
Fone:   216.796.3163                       Research Center
Fax:    216.796.3304                       142 Goodyear Boulevard
					   Akron, OH   44305
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For a successful technology, reality must take precedence over
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