Subject: Chemical shielding: sigma_d and sigma_p

From: Chun Chung Chan <[email-masked]>
Date: Sun, 8 Apr 2001 16:28:29 +0200 (MES)

Dear All,

I know that the division of diamagnetic (sigma_d) and paramagnetic
(sigma_p) parts of chemical shielding tensor really depends on the choice
of gauge origin, and only sigma_d + sigma_p is gauge invariant. 

I suppose that there is only one gauge origin for a molecule. Then, is it
incorrect to compare the diamagnetic parts of the same nuclei located at
different parts of a molecule? In many commercial programmes, we can find
the listing of diamagnetic and paramagnetic contributions in GIAO or IGLO
calculation. Could anybody please tell me why it remains sensible to list
sigma_d and sigma_p separately? When we use GIAO or IGLO to calculate
chemical shielding tensor, where is the gauge origin?

Furthermore, I have heard that sigma_d is always positive. It seems ok
when the nuclear position is chosen as the gauge origin. However, what
would happen in a molecule? Is it possible that the sigma_d of a
particular nucleus could be negative? I suppose if sigma_d + sigma_p is
gauge invariant, I can always find a gauge origin where sigma_d would
become negative. Please let me know if I am mistaken.

Many thanks for your comments,
Jerry

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* Jerry Chun Chung CHAN                     [email-masked] *
* Universitaet Muenster	                                         *
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