Subject: Geometry issues for semiempirical methods

From: <[email-masked]>
Date: Fri, 14 Jan 94 14:45:54 -0800

Semiempirical methods, specifically MNDO/AM1/PM3, do rather poorly on
calculated rotational barriers for molecules such as 1,2-Dihaloalkanes,
1,2-Dimethoxyethane, and even ethane itself.  Usually the rotational
barrier between syn-trans is underestimated by as much as 50%.

Has anyone else documented this sort of problem (I presume this problem
is fairly well known).

Is anyone working on (or published) modified model Hamiltonians that
can treat this issue?  

I wonder how SAM1, Walter Thiel's new MNDO method, or SINDO might
treat this...any thoughts?

I'd be happy to summarize if you email directly to me.

Mark Thompson

ps.  There is a recent paper on 1,2-Dihaloalkanes by Dave Dixon and
coworkers that discusses this issue.  (J. Phys. Chem. vol 96, p 10740,
1992).

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Mark A. Thompson                    
Sr. Research Scientist              email:  [email-masked]
Molecular Science Research Center   FAX  :  509-375-6631
Pacific Northwest Laboratory        voice:  509-375-6734
PO Box 999, Mail Stop K1-90
Richland, WA.  99352

Argus available via anonymous ftp from pnlg.pnl.gov (130.20.64.11) (in the
argus directory).  Download the README file first.
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