The energies of the 37 observed levels of the 4ƒ<sup>3</sup> configuration of Pr III have been calculated with an rms deviation of ±29 cm<sup>-1</sup> by use of 13 adjustable parameters, including 5 parameters which account for nonlinear configuration-interaction effects. The physical significance of the nonlinear parameters is discussed in terms of the relative roles of various mechanisms of configuration interaction.
K. RAJNAK, "Configuration Interaction in the 4ƒ3 Configuration of Pr III," J. Opt. Soc. Am. 55, 126-132 (1965)