Subject: More flame? Was: Semiempirical parameterization ...

From: JING KONG <[email-masked]>
Date: Tue, 19 Apr 1994 23:38:50 -0300

Dear netters,

	The discussion raised from parameterization of semiempirical 
methods seemingly becomes more or less about the philosophies behind
different approaches.  I would like to add some comments to what Holder, 
Letner and Burkhart wrote. But most time I will use E.R. Davidson's words 
since he is one of the few who made major contributions to the mainstream QM 
ab initio methods (and thus is more authoritive :-).)  His words are in 
quotation marks.

Holder wrote:
>  I can safely say that MM has nothing to do with chemistry,
> but is more or less a fit to a convenient set of functions.  It
> does have chemical validity in that it gives us good answers to some 
> questions.

Letner replied,
>However MM force fields do have terms that model chemical phenomenon. 
>Examples include bond angles, bond lengths, etc.  Granted, these are

Bond angles and bond lengths were in Chemistry many, many years ago
(100?). Can MM say more about bond angles and bond lengths than numbers?

Holder wrote:
>   QM is an entirely different situation, in that the actual arbiters of 
> chemistry, the electrons, are treated directly by mathematical functions.
> The chemistry come OUT of a model that describes the electrons.

Unfortunately, not much chemistry have come out so far from mainstream QM 
methods. "Finally, with most modern programs, the user is left with exactly 
the same information that Pauling started with 60 years ago."

Letner wrote:
>	Further, is not any mathmatical fomalism a model?  This is to
>include QM, a more detailed and complicated model of atomic events.  Yes,

I agree. QM approach (I mean mainstream such as ab initio and DFT) 
based on the same philosophy as MM: use mathematical models derived 
approximately from physical laws to produce numbers. Does this approach work? 
"It will be noticed that the Nobel prizes in quantum chemistry have gone
to Pauling, Mulliken, Hoffmann, Fukui, and Lipscomb. All of these people 
are noted for the models of the chemical bond that they have developed and 
used to explain important chemical phenomena. Even in the computer age, 
they focused on incorrect, but useful, empirical wavefunctions. The insights 
that they obtained were not found on any computer pritout."  

Simply, Chemistry is neither Physics nor Mathematics.

One may hope that QM would work quantitatively. "Atomic orbitals do not 
work. The error in a wave function is largest where the wavefunction is largest.
The computer cost, at fixed accuracy, grows like N!."

I feel that an optimal solution should be good both quantitatively and
qualitatively. 

All of these are concerned only by method developers and those who want to 
understand more about the chemistry part of the nature.  For those practical 
users, take what
Burkhart wrote:
>The faith comes in the numbers...the better the numbers, the
>better the faith. No more, no less.

And, be happy.

Jing Kong
Dalhousie

Last quote: "Computers do not solve problems, people do." 
in Reviews in Computational Chemistry, Kenny B. Lipkowitz and Donald B. Boyd 
Eds., VCH 1990.