The Four Vector Problem and Its Application to Energies and Intensities in Platinum-Like Spectra

The theory of two-electron systems involves both the electrostatic interaction between the electrons and the magnetic interaction between the spin and orbital moments of each electron. The inclusion of this latter effect requires the knowledge of the matrix of $a_1{\mathrm{l}}_1·{\mathrm{s}}_1+a_2{\mathrm{l}}_2·{\mathrm{s}}_2$. This matrix has been given by Johnson, who obtained it by the method of noncommutative vector algebra. An outline of an alternative method based on the correspondence principle is given here because of its simplicity.