Subject: NMR Chemical Shielding Anisotropy

From: <[email-masked]>
Date: Tue, 22 Apr 1997 15:41:38 +0200 (DFT)

Hi,

I am computing chemical shielding tensor for various molecule.
Once you diagonalyze this tensor you can expressed the 
isotropic chemical shield as the average of the three eigenvalues
(Txx, Tyy, and Tzz). You can also compute the anisotropy, which
is the difference between the larger eigenvalue (generally called
Tzz) and the average of the two other. 

	Iso = 1/3 (Txx + Tyy + Tzz)
	Ani = Tzz - 1/2 (Txx + Tyy)

Everything is fine with me when all three eigenvalues have the same signe.
Problem arises when one eigenvalue is negative.
For exemple, here are three eigenvalues : -76.8050  6.2091  62.8089
If you consider that Tzz = 62.8089 you'll find an anisotropy of
	62.8089-(6.2091-76.8050)/2 = 98.1069
However, if you consider that Tzz = -76.8050, (In absolute value 76.8050 is
larger than 62.8089.) the anisotropy = -111.3140

My question : which one is the good choice ?
I already know that G94 use the first definition.

Thank you in advance for your advice. I'll summarize.

						...Xav

[email-masked]