Xref: utzoo sci.physics:3291 sci.misc:1407 sci.research:369
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From: lim@cit-vax.Caltech.Edu (Kian-Tat Lim)
Newsgroups: sci.physics,sci.misc,sci.research
Subject: Re: atomic simulation software ...
Message-ID: <6307@cit-vax.Caltech.Edu>
Date: 29 Apr 88 08:48:14 GMT
References: <203@heurikon.UUCP> <4864@cup.portal.com>
Reply-To: lim@cit-vax.UUCP (Kian-Tat Lim)
Organization: California Institute of Technology
Lines: 37

In article <4864@cup.portal.com> doug-merritt@cup.portal.com writes:
>Ray Lampman writes:
>
>>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?
>
>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.
>

Accurate, ab initio (from first principles) simulations of molecules as large
as a couple of dozen (light) atoms are *not* that difficult.  The only
problem really requiring empirical judgement is where to cut off your basis
set of orbital functions.  See, for example, the GAUSSIAN series of programs
developed by Pople at CMU and Hehre at UC Irvine, or the generalized valence-
bond implementations from my adviser, Bill Goddard here at Caltech.  These
programs can produce structures and energies that in some cases been more
accurate than contemporary experimental results.

Now, admittedly, these are not *complete* solutions to the relevant QM
equations [when they are variational, they don't have the lowest possible
energy]; but they are adequate for most uses.  I believe that Aron Kupperman
here has produced provably correct solutions for the "single and dual
hydrogen atom systems" mentioned above, "with extreme difficulty," though I
am not as familiar with his work.  However, this is a level of detail that is
unnecessary in almost all cases.

Semi-empirical and empirical (force-field) methods allow treatment of much
larger systems (I work with proteins of a hundred or so amino acids), but with
less accuracy and more caveats.

--
Kian-Tat Lim (ktl@wagvax.caltech.edu, GEnie: K.LIM1)