Subject: Shielding is a ground state property

From: Jerry C C Chan <[email-masked]>
Date: Tue, 5 Mar 1996 14:47:37 +0800 (HKT)

Dear Netters,
	
	After going through some articles, I have a thinking about the
future aspect of the calculation of shielding constant and I would like to
have your comments.

	Currently, magnetic effect on a molecule is treated via
perturbation theory in density functional approach.  The calculation of
shielding constant involves excited states as in CHF theory.  Very
recently it is published the theoretical framework for incorporating
either the magnetic field (suggested by C.J. Grayce and R.A. Harris at
Berkeley) or the current density (by G.  Vignale and M. Rasolt) into the
exchange-correlation (XC) functional.  Although an explicit XC functional
which can be used for the magnetic-field density functional theory or the
current density functional theory is not yet realized, the equations of 
shielding calculation for both theory have been published (Grayce and 
Harris for MDFT; Colwell and Handy at Cambridge for CDFT).

	My speculation:

	Once the XC functional for either theory appears, diamagnetic
shielding would become an eigenvalue problem [assume the guage origin is
chosen at the nucleus].  The paramagnetic shielding constant is just
evaluated in the same way as the diamagnetic term in CHF theory, i.e.
evaulation of the triple product of the ground state bra, the operator 
of the vector potential of the electrons exerted by the magnetic moment 
of the nucleus and the ground state ket.

	It means that the time required for shielding calculation in MDFT
or CDFT approach is of the same order of magnitude as the diamagnetic
terms in CHF, provided that the SCF density can be evaluated as easy as in
HF approach.  Since excited state is no longer required for the
calculation, shielding constant should then become a ground state property. 
Accuracy of the shielding results depend solely on the quality of the XC
functional.  Similar sayging is applicable to all second order 
observables with respect to magnetic field.

Cheers,
Jerry C.C. Chan