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 ≈1 eV, the s–s contribution being the larger. The p–p energy is ≈1/3 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.