Subject: MNDO vs. AM1

From: <[email-masked]>
Date: Fri, 21 Apr 1995 16:23:22 CST

Netters,

A question was recently posted as to the difference between MNDO
and AM1.  I'll be happy to give my take on this.

1.  The basic difference between the two methods is the addition of 
Gaussian functions to some of the core repulsion functions (CRFs) 
for certain elements.  These functions have a position, a width, 
and an intensity, all of which can be taken as parameters.  They 
were initially intended to adjust the SHAPE of the CRF so that it 
would more closely correspond to "reality", whatever that is.  In 
essence, they were used as patches for specific types of systems 
that could not be handled in general parameterization without 
disrupting other chemically important items.  In my experience, 
the Gaussians were added one at a time and with great deliberation, 
and only after the other parameters were pretty much settled on.  
This is because they could potentially have such a large effect
on the chemistry of the elements.

An example of this are the Al-Cl systems for AM1.  When I was
finishing parameters for Al in AM1, these systems would simply not
come out correctly.  A carefully placed Gaussian solved the problem.
Another example of a "problem" Gaussian function is the one found
at about 3.0 angstroms in the phosphorous parameters for AM1.
This Gaussian can cause extensive problems when there are TWO P
atoms at about this distance in a compound.

All in all, Gaussians MUST be used with extreme care and anytime
one is added to repair a problem, it will almost certainly cause
another.  As with all things, it is a matter of trade-offs.
The above discussion has led to my conclusion (tongue firmly in
cheek) that Gaussians are "evil."  I think I would amend that to
be a "neccessary evil."

2.  Another difference that must be mentioned is additional flex-
ibility in the parameters for the lighter elements in the case of
AM1.  For instance, for most of the lighter elements (rows 1 and 2)
only one zeta value (exponent of the slater orbitals) and one beta
value (resonance) was used for both s and p orbitals in the valence
shell within MNDO.  This approximation was abandoned for the heavier
elements in MNDO and completely for AM1.  This assumption was made
in the case of MNDO for computational efficiency.

3.  The third difference is the added computational power available
for parameterization searches when AM1 was parameterized.  Much more
extensive grid searches and trial parameter examination could be
carried out more quickly as timne went by.  Most of the AM1 parameter
sets are substantially different than the MNDO ones indicating that
definitely different parameter minima were located.

All in all, the methods are basically the same in theory differing
only in the details.  The same can be said about PM3, in that it
was a different parameterization method, not a different theory.
For most cases, the performance of AM1 is considered to be generally
superior to MNDO.

 Regards,  Andy Holder

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UUUU  UUU MMM   MMKK KKKK   CCCC   |           ANDREW J. HOLDER
 UU    U   MM   MMK   K    CC  CC  | Asst. Prof. of Comp./Org. Chemistry
 UU    U   MMM M MK KK    CCC      |          Dept. of Chemistry
 UU    U   M MM  MK   KK   CC  CC  |  University of Missouri-Kansas City
  UUUUU   MMM M MMKK   KK   CCCC   |         Kansas City, MO  64110
                        KK         |          [email-masked]
                          K        |  (816) 235-2293, FAX (816) 235-5502
 
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