Subject: Re: CCL:EHM TODAY--OPINIONS

From: Carlo Mealli <[email-masked]>
Date: Thu, 20 Nov 1997 09:25:30 -0500 (EST)

On Wed, 12 Nov 1997 16:22:51 -0500 (EST) "E. Lewars" 
<[email-masked]> wrote:
>
>Wed, 1997 Nov 12
>
>>From E. Lewars
>To:  CCL
>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?
>Thanks
>  E. Lewars
>================

   Almost as an hobby and also thanks to the help of some enthusiastic 
collaborators (first of all Dr. D.M.Proserpio), I have long worked to develop 
a handy graphic interface (package CACAO) to the numerical results of ehmo 
calculations (the basic programs of Roald Hoffmann at Cornell). 
I am very much aware of the shortcomings of the method in terms of highly 
reliable quantitative results. Mainly, the problems arise from neglecting the 
inter-electronic terms. Under these circumstances it is impossible to trust 
blindly a quantitative response associated, for example, with geometry 
optimization or with the energetics of an electronic transition . 
   Still most of the times "the trends" are correct and there is a lot to learn from 
trends! If used "cum grano salis" (i.e. always referring to the common 
concepts of chemical bonding, stereochemistry and reactivity), the method 
provides a lot of quick, useful and reliable information. The rigourous 
subdivision of the MOs in terms of symmetry properties (remember the 
relevance of the Conservation of Orbital Symmetry ideas expressed by Roald 
Hoffmann) is dense of chemical meanings. Thus, if a low symmetry molecule is 
analyzed starting from the conformer with the highest possible symmetry, the 
evolution of the MOs and the redistribution of the electrons in them do almost 
invariably point toward the correct solution . Moreover it is clearly seen where 
the electronic problems arise from in the conformers with higher symmetry. In 
other words, the Perturbation Theory works well and the ehmo method helps 
to interpret it beautifully. 
   Nowadays with the ubiquitous PCs, it is fast and easy to perform these 
calculations at will and to explore many different deformational trends and/or 
reaction pathways. The graphics more than the pure numbers are the helpful 
interpretational tool and also the non-specialists (the experimental chemists, 
the teachers, the students, etc.) can now seek a suited MO description of the 
chemical bonding in a molecule or series of molecules.   But there is a good 
point to continue to exploit the ehmo method also for those who perform high 
level theoretical studies. My viewpoint is that ehmo 
cannot be substitutive but is a coadjutant of the latter methods most of which 
(from AM1 on) provide no interpretational tools, no eventual insight to the 
user. As in an explorative approach, a preliminary ehmo analysis sheds light in 
the field where the higher level computations are meant to be 
performed. Moreover, I notice myself from comparisons that the basic 
quantitative results are very often consistent. For example, the frontier MOs 
from the ehmo and those from accurate ab-initio methods correspond in 
energy order, symmetry and composition. When this is the case, one can 
strategically use the ehmo method to interpret to basic underpinnings of the 
given quantitative result. Beside the list of geometrical parameters of an 
optimized geometry, the chemist needs to know the electronic factors which 
affect the latter in order to understand the role, say, of some substituents. The 
ultimate goal of theory is to favour the planning of new experiments.
…Much chemistry has been interpreted in the past 20-30 years thanks to the 
ehmo method because it is a powerful telescope to see Perturbation theory at 
work. In my opinion ehmo cannot be dismissed as obsolete especially now 
that personal computers allow to construct a MO description of chemical 
bonding almost as easily as the Valence Bond description is constructed by 
using paper and pencil.

Dr. Carlo Mealli

==========================================
Dr. Carlo Mealli
ISSECC - CNR, Via Nardi 39, 50132 Firenze, Italy
Tel 0039-55-2346653, Fax 2478366
e-mail: [email-masked]
==========================================