Abstract: | Analytic properties of charge densities associated with singlet and triplet electron pairs, ρ0( r ) and ρ1( r ), are presented. In an N‐electron system with total spin S, distributions ρ0( r ) and ρ1( r ) are independent of the spin projection quantum number M (spin rotation invariance), as opposed to the usual spin‐up and spin‐down electron densities, ρα( r ) and ρβ( r ). We derive equations showing that in the case of a wave function which is a spin‐eigenfunction, ρ0( r ) and ρ1( r ) are linear combinations of the total charge density ρ( r ) and the uncompensated spin density ρs( r )=[ρα( r )−ρβ( r )]/2M. For a wave function which is not an eigenfunction of $mathcal{S}^{2}$, no such relationship exists. In a related discussion, a definition of the high‐spin solution corresponding to a given spin‐unrestricted Hartree–Fock wave function is proposed, and a notion of effectively unpaired electrons is introduced. The distributions ρ0( r ) and ρ1( r ) are shown not to be invariant under spin coupling between isolated systems. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 651–660, 2000 |