Subject: Summary: Experimental or Optimized Structures

From: Jerry C C Chan <[email-masked]>
Date: Thu, 30 May 1996 11:59:27 +0800 (HKT)

Dear Netters,

	Two weeks ago I posted a question on the following issues:

1.  Absolute shielding is considered as the best measure of the quality of
calculated shielding tensor components.  Why not chemical shift anisotropy
and eta? 

2.  Since the experimental CSA and eta "contain" the effects of the
environment, should one choose the experimental geometry rather than
trying to optimized the geometry of the species when the absolute 
shielding scale is not available.

	The enlightening responses from Robert and Georg are greatly
appreciated.  Below please find the responses received.

Jerry,
The Chinese University of Hong Kong

*****************

My bet is that most will tell you to use the experimental geometry but I   
would advocate a high level ab initio optimized geometry. X-ray results   
are influenced by crystal packing which distort the geometry to some   
extent. You usually do the NMR shielding calc in vacume so why not have a   
vacume geometry. Thsi should be obtained from a decent search and   
minimization at a theoretical level comparable to that at which you will   
do the NMR shielding calc at.
  Thank-you
  Robert A. Kirby
  BioChem Therapeutiques Inc.
  275 Armand-Frappier Blvd.
  Laval, Quebec,
  H7V 4A7
  [email-masked]

*****************  

> 1.  Absolute shielding is considered as the best measure of the quality of
> calculated shielding tensor components.  Why not chemical shift anisotropy
> and eta? 

Those two are undoubtely also good measures for the quality of your
calculation -- if you have the experimental results. 
Shieldings are often used because there is always
the possibility that some systematic error remains hidden
in relative shift if it happens to cancel out.

> 2.  Since the experimental CSA and eta "contain" the effects of the
> environment, should one choose the experimental geometry rather than
> trying to optimized the geometry of the species when the absolute 
> shielding scale is not available.

If you have a GOOD experimental structure, than it is probably preferable.
I stress the word "good" here since the shielding is extremely sensitive
to small changes in the geometry (this in turn is just one of the reasons
why experimentalists do NMR measurements).
However, in many cases of interest, you don't have such structures, or
not for the whole set of molecules that you are studying. In these cases,
calculated structures are your choice. Some people use exclusively optimized 
structures since this provides a consistent picture.

Yours, Georg

P.S. I am looking forward to your summary.

==============================================================================
Georg Schreckenbach                      Tel: (Canada)-403-220 8204
Department of Chemistry                  FAX: (Canada)-403-289 9488
University of Calgary                    Email: [email-masked]
2500 University Drive N.W.,  Calgary,  Alberta,  Canada,  T2N 1N4
==============================================================================

>From [email-masked] Wed May 29 17:34 EDT 1996
Date: Wed, 29 May 1996 16:25:02 -0500
From: Boyd <[email-masked]>
Subject: FREE BOOKS
To: OSC CCL <[email-masked]>
Content-Length: 1373
Status: R

Dear CCLers,
	We are pleased to announce the 1995 winners in the contest 
offered on the "Reviews in Computational Chemistry" home page:  Prof. 
Christopher J. Cramer (University of Minnesota) and Prof. Helder M. 
Marques (University of Witwatersrand).  Each had 6 or more citations to 
chapters from "Reviews in Computational Chemistry" in their 1995 
journal articles.  Each is being awarded a free volume of "Reviews in 
Computational Chemistry".  Congratulations, Chris and Helder!
	We thank everyone who entered the contest this spring.  
Remember that awards will made next spring based on citations to "Reviews in
Computational Chemistry" in 1996 journal articles.
	Another give-away currently being offered on the "Reviews in 
Computational Chemistry" home page is for teachers who use the books 
in conjunction with their classes.  Details are given on the home page.
	We wish everyone good luck in all the give-aways.

Sincerely,
Don
Donald B. Boyd, Ph.D.
Research Professor of Chemistry
Co-Editor, REVIEWS IN COMPUTATIONAL CHEMISTRY
Department of Chemistry
Indiana University-Purdue University at Indianapolis
402 North Blackford Street
Indianapolis, Indiana 46202-3274, U.S.A.
Telephone 317-274-6891
Facsimile 317-274-4701
Internet [email-masked]
REVIEWS IN COMPUTATIONAL CHEMISTRY Home Page on the World Wide Web 
URL http://chem.iupui.edu/~boyd/rcc.html

>From [email-masked] Tue May 28 13:14 EDT 1996
From: [email-masked] (David Moses)
Subject: G94 available on new SGI platforms
To: [email-masked]
Date: Tue, 28 May 1996 11:23:22 -0400 (EDT)
Content-Length: 2336
Status: RO

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