Correlation energies of light atoms related to pairing between antiparallel spin electrons

Early theoretical work emphasized the gross trend of the total correlation energy of neutral atoms with atomic number Z. This was later improved quantitatively, by focusing on the number of pairings between antiparallel-spin electrons in the same main shell (i.e., K, L, M... shells). Here, this viewpoint is pressed, and correlation energies associated with s-s, s-p and p-p pairings are extracted. The S-S energy turns out to be rather insensitive to a change from K to L shells, but a small increase is found to occur in the M shell. Also, the s-p and p-p pairing energies show a small increase from the L to the M shells. The s-s and s-p energies are approximate to1 eV, the s-s contribution being the larger. The p-p energy is approximate to1/3 eV. The effect of removing or adding one electron to the neutral atom has also been analysed. Wavefunction overlap and wavefunction localization appear to control the values of the pairing energies.