Correlation Energy for Atomic Systems

The correlation energies are estimated for the isoelectronic series of atoms with 2 to 10 electrons in the ground state and in the excited states of the lowest electronic configuration. The obtained values of the correlation energy point out clearly the importance of precorrelation energy, i.e., the energy difference between Hartree and Hartree—Fock energies, the quasi-additivity of 1s, 2s, 2-, ··· intra- and intershell correlation, the relative importance of spin-multiplicity correlation and total angular-momentum correlation, and the Z effects on correlation. The estimate of the correlation energy makes use of energy data for the relativistic energy: the accuracy of the relativistic energy computations is discussed. The 2 to 10 electron correlation energies were used as a basis to estimate the 11 to 18 electron correlation energies and to draw general conclusions on the value of the correlation energy for any atomic system.