  Pseudocontact shifts are traditionally described as a function of the
anisotropy of the paramagnetic susceptibility tensor, according to the
semiempirical theory mainly developed by Kurland and McGarvey (R.J. Kurland and
B.R. McGarvey, J. Magn. Reson. 2, 286 (1970)). The paramagnetic susceptibility
tensor is required to be symmetric. Applying point-dipole approximation to the
quantum chemistry theory of hyperfine shift, pseudocontact shifts are found to
scale with a non-symmetric tensor that differs by a factor g/ge from the
paramagnetic susceptibility tensor derived within the semiempirical framework.
We analyze the foundations of the Kurland-McGarvey pseudocontact shift
expression and recall that it is inherently based on the Russell-Saunders (LS)
coupling approximation for the spin-orbit coupling. We show that the difference
between the semiempirical and quantum chemistry pseudocontact shift expressions
arises directly from the different treatment of the orbital contribution to the
hyperfine coupling.
