A relationship between correlation energies and sizes: the series of beryllium- and neon-like ions

A simple empirical relation for the estimation of the correlation energy (Ecorr) is found to be valid for the isoelectronic series of Be- and Ne-like ions to a very good accuracy. Although originally such a relation was proposed for the He-like ions, the present results indicate that it is applicable for layer systems as well. Thus the interelectron pair correlation in systems such as Be or Ne does not affect adversely the accuracy of the relation:[Formula: see text]where the denominator is the size [Formula: see text] weighted by the square of the effective nuclear charge Zeff.