CCL: Hyperfine and spin density

["Frank Neese" <Frank.Neese _ cec.mpg.de>]

 Sent to CCL by: "Frank  Neese" [Frank.Neese . cec.mpg.de]
 It is a common misconception that spin density (a three dimensional function of
 space) is confused
 with spin population (a more or less arbitrary assignment of integrated spin
 density onto atoms). While
 ideally one wants the s-orbital spin population to be proportional to the spin
 density at a given
 nucleus, in reality it is more complicated - it really matters which basis
 functions do make up the spin
 density. Different s-orbitals differ strongly in their value at the nucleus and
 it depends strongly on the
 specifics of the bonding which ones are selected by the variational principle to
 contribute. Hence, I am
 not at all surprised to see a substantial change in the hyperfine while the spin
 population is barely
 changed.
 Here is a reference to a paper that analyzes the lack of proportionality of spin
 population and hyperfine
 coupling in a series of Cu(II) complexes with nitrogen ligands.
 F. Neese, J. Phys. Chem. A. 105 (2001) 4290.
 Good luck!