Subject: CCL:EHM TODAY--OPINIONS

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Thu, 13 Nov 1997 12:34:52 +0100

Wed, 1997 Nov 12, E. Lewars:

> Subj: Extended Hueckel Today--opinions
> 
> Hello,  What do people out there in netland think of the *current* status of the
> extended Hueckel method that was popularized by Roald Hoffmann, starting
> ca. 1963?   It was, I think, the first generally applicable method, in the
> sense that it was not limited to planar pi electron arrays and could in
> principle perform geometry optimizations.  Is it the general view that it is
> essentially obsolete, having been displaced by more sophisticated methods like
> MNDO and its decendants AM1 and PM3?  Does it have some advantages over AM1
> and PM3?

EHM does have one fundamental advantage over AM1, PM3 and other 
NDO-type theories: It is based on a proper treatment of atomic 
orbital (AO) overlap and thus includes terms that are neglected in 
NDO theories.  For example, the repulsion between closed shells is a 
second order overlap effect, that is not reproduced by NDO theories.  
This leads to situations where EHM is superior to NDO theory. e.g., 
Tetrahedron 39, 3345 (1983). 

Jens >--<
                             
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JENS SPANGET-LARSEN          Phone:  +45 4674 2000  (RUC)
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