Subject: EXTENDED HUECKEL--SUMMARY AND THANKS VERY MUCH

From: "E. Lewars" <[email-masked]>
Date: Sun, 16 Nov 1997 18:18:32 -0500 (EST)

Sunday Nov 16

>From E. Lewars
To:  CCL
Subj: Summary of Extended Hueckel Today--opinions

This is a summary of the answers I got to my question (below).
Thanks very much indeed to all who helped.
=========

The Question (Wed 1997 Nov 12):

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?

Thanks
  E. Lewars
================
1997 Nov 14

Summary of responses (I hope I haven't missed anyone):

       OPINIONS ON CURRENT STATE OF EHM

#1  [email-masked]
#2  [email-masked]
#3  [email-masked]
#4  Thomas Albright, U of Houston

#1              13-NOV-1997 08:14:25.28

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 Spanget-Larson, Roskilde U, Denmark
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#2               12-NOV-1997 18:00:12.43

Personally, I find it still valuable, since it's generally consistent, 
includes support for elements that PM3, etc, don't, and has been
generalized to the solid-state.  It's also conceptually simple, serves
as a good tool for a first pass at a problem, and is cheap to run,
allowing qualitative results to be obtained for large systems in minimal
time.  If the limitations are taught first, it serves as an excellent tool
for teaching basic modeling to students without overwhelming them in the
technical details of ab-initio (or even better semi-empirical) methods.

My preference, due to an interest in transition-metal systems, is to use
Fenske-Hall, but that method is regrettably much less available, and has
not been generalized to periodic systems.

Fred P Arnold, Advanced Research Systems, Chicago
==

#3

Does it have some advantages over AM1
and PM3?
--- end of quote ---
EHMO has the obvious practical advantage of requiring far fewer parameters than
either AM1 or PM3. Therefore, it has been extended to many more elements, and
it is easier to "see" where the computed results come from in EHMO.

The disadvantages of EHMO are many: one-electron method (no accounting of ee
interactions or spin), can't predict energy differences, can't predict
geometries (you say it can "in principle", but EHMO predicts that H2 should be
a "superatom"). So its hard to see why anyone would choose to use EHMO if
another method were available.

Alan Shusterman, reed College, Portland, Oregon

====

#4

 .....I wouln't want to use EHT to "explain" vibrational spectra, but it would
and has been extremely useful in understanding molecular shapes.  I will
return to this point in a minute.  problems that relate to angular variations
are given quite well by EHT and this has been extensively documented in the
literature  Too much of what I see in the CCL postings relate to blind number
crunching.  If that is what one wants to do then I sincerely doubt that the
EHM will be useful.  But why should one do this?  I strongly believe that a
theoretician should either be predictive (and not in a derivative or trivial
way) or offer theories about physical phenomena.  Extended Hueckel performs
admirably in the especially latter area.

Thomas Albright, Professor and Graduate Chair, U of Houston
==============