Xref: utzoo sci.physics:3279 sci.misc:1399 sci.research:364
Path: utzoo!mnetor!uunet!portal!cup.portal.com!doug-merritt
From: doug-merritt@cup.portal.com
Newsgroups: sci.physics,sci.misc,sci.research
Subject: Re: atomic simulation software ...
Message-ID: <4864@cup.portal.com>
Date: 27 Apr 88 20:41:48 GMT
References: <203@heurikon.UUCP>
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Ray Lampman writes:

>I'm looking for a software package which will be able to help project the
>physical properties for elemental and multi-atomic structures. [...]
>I have read my own request and am torn between laughter and seriousness.
>I believe the quantum theory is capable of modeling chemical bonds.
>But do we have a theory which models the interactions between radiation
>and matter? If we have sufficient theory, do we have the computing power
>to complete a valid simulation? Are we missing any parts of this puzzle?

Your doubts are very well founded. Yes, the theory is (probably) adequate.
No, we don't have anything even *close* to the computing power necessary.
Simple simulations of single and dual hydrogen atom systems have been
carried out with extreme difficulty. Simulations of anything more complex,
like say simple sugars, alcohol, methane, etc are totally out of the
question.

Particle physics has a similar problem with computational complexity
of Feynman diagrams (see for instance the very readable QED by Feynman).

In general, chemistry is an empirical field *aided* by narrow applications
of quantum mechanics. There is currently no hope of predicting general
chemical properties wholly from first principles.

I talked this over with a friend who's doing research in these areas,
and he says that all they can do is use empirical approximations, and
keep looking for new theories. That's not to say he implied anyone thought
that a theory would be invented that allowed total predictions, though
(quite the contrary).

On yet another related topic, it turns out that weather prediction is
a far more mature field than most people think. The theory works fine,
given limitations on the data collection process (precision of measurements,
and the resolution of the collecting grid).

The principle reason why forecasts are inaccurate beyond a few days out
is again because of computational complexity. It's a nonlinear dynamical
system which cannot be solved in closed form; it must be simulated one
step at a time, and initial inaccuracies of one part in a trillion
turn into 100% errors very quickly. See Chaos by James Gleick (also
very readable).

Back to your original request, depending on your purpose, you might
do just as well with a package that is based on empirical data and
take it from there. There's lots of molecular modeling packages
available for various purposes. I'm not particularly familiar with
the details. There was a Scientific American article on the subject
sometime in '86.


      Doug Merritt        ucbvax!sun.com!cup.portal.com!doug-merritt
                      or  ucbvax!eris!doug (doug@eris.berkeley.edu)
                      or  ucbvax!unisoft!certes!doug