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From: chas@gtss.UUCP (Charles Cleveland)
Newsgroups: sci.physics,sci.misc,sci.research
Subject: Re: atomic simulation software ...
Message-ID: <247@gtss.UUCP>
Date: 29 Apr 88 03:26:04 GMT
References: <203@heurikon.UUCP> <4864@cup.portal.com> <6567@lll-winken.llnl.gov>
Reply-To: chas@gtss.UUCP (Charles Cleveland)
Organization: Georgia Tech School of Physics
Lines: 51

In article <6567@lll-winken.llnl.gov> colvin@mozart.llnl.gov.UUCP (Mike Colvin) writes:
)In article <4864@cup.portal.com> doug-merritt@cup.portal.com writes:
)>
)>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.
)
)  Actually this is not entirely correct.  There is now a growing field 
)of chemistry known as ab initio quantum chemistry that involves the
)numerical solution of the molecular Schrodinger equation to calculate
)chemical properties.  Of course these methods yield only approximate

Well this is entirely charming :-).  Not to mention encouraging.  I
applaud all the people you mention in the text I have deleted and
professionally respect the quality of their work.  But totally apart
from quantum mechanical questions, we can't even take an arbitrary
simple potential (of the order of Lennard-Jonesium, for example) and
predict what crystal structure it prefers.  Even classically.  At zero
degrees.  Of course given two structures, we can tell which is lowest
in energy, but that's a different question.  The real problems is to
find the structure without restricting the problem to certain preconceived
possibilities.  And not everything is crystalline.  Can we predict
quasi-periodic structures?  What about the possibility of an amorphous
ground state?  And what about finite temperatures?  The original poster
wanted to know about phase diagrams.  The mind boggles.

I would say the poster you responded to stated the situation as per
the application of quantum mechanics to general chemical questions
far more accurately than you did, but who the hell cares?  The
calculation of electronic structure for a given arrangement of nuclei
is the easy part of the original poster's pipe dream, although it's
hard enough.  At least the computational rules are clear, and the
approximations more or less well understood.

The problem is not merely computational.  In the case of finding the
structure which minimizes the energy of a set of atoms interacting
with some given set of potentials, we don't even know how to express
the problem in a way conducive to its general solution.  Of course if
we did, the problem of finding the global minimum would still be of
staggering computational difficulty, simply by virtue of the vast number
of degrees of freedom in the calculation.

Disclaimer:
If there is recent literature which I seem to be unaware of, I would
appreciate being made aware of it.  Nothing could please me more than
to learn that my comments here are dated.
-- 
-Life would be so much easier if we could just look at the source code.-

Charles Cleveland    Georgia Tech School of Physics    Atlanta, GA 30332
UUCP: ...!gatech!gtss!chas         INTERNET:  chas@ss.physics.gatech.edu