Correlation Energy ofK-Shell Electrons

An estimate of the correlation energy of $K$-shell electrons for ${\mathrm{F}}^{-}$, Ne, ${\mathrm{Na}}^{+}$, ${\mathrm{Cl}}^{-}$, Ar, and $K^{+}$ has been obtained by analyzing $K$-shell ionization energies. The relativistic correction has been determined by also considering the orbitals outside the $K$ shell. In addition, the Lamb-shift correction has been evaluated. The correlation-energy values estimated generally agree with the results of other independent methods of obtaining correlation energies.