Subject: "G2" for larger systems....

From: <[email-masked]>
Date: Tue, 27 Aug 1996 04:39:19 -0400 (EDT)

Jeremy asked the following questions...

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Are there somewhat less expensive compound methods (G2) which are
recommended?
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I'm not particularly experienced in this issue, but there are several
related works in our group (K. Morokuma), which might provide some useful
info.

First of all, there are always those scaling approaches, which works
rather well in many cases, including transition metal chemistry. One of
them is the so-called PCI-80, proposed by M. Svensson and P. Sigbahn.

Secondly, some of our group memebrs are working on the so-called IMOMO
method, which is basically treating the active part with high accuracy,
and the rest with relatively low accuracy. Indeed, in most chemical
reactions, correlation effects are rather localized, the inactive part are
either correlation-insensitive, or effects simply cancels out if one
considers the relative reaction profile. In those cases, one can do really
high level calculations such as G2 on the active site. For example, people
in our group did some benchmark calculations on a series D-A reactions(
with large substituents) and found really good agreement with several gas
phase exp. results. They've also done some systematic studies on the S/T
transition energies for a series of large molecules,and again found
promising results. I guess the merit of those approach is, one does't
really need any new code to do the calculation, (provided that u have a
decent geometry, people in our group are also doing geom. optimization
using IMOMO/IMOMM). All u have to do is, do a few single points calc. at
different levels, and put them together. Sophisticated mathmatics doesn't
always mean better results. (although personally I like rigorous math
better:-)

Any way, that's what I can think of and what I'm relatively familar with.
If u r interested, u can contact me or [email-masked] for
some references.

Hope this will help in some way..

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Qiang Cui
Dept. of Chem. Emory Univ.         508 Webster Dr. Apt.#2
Atlanta, GA 30322.                 Decatur, GA 30033.
(404)-727-2381                     (404)-636-6149

http://tswww.cc.emory.edu/~qcui
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