The positions of 38 observed levels of the 4ƒ<sub>3</sub> configuration of Pr III were calculated with a rms error of ±168 cm<sup>−1</sup>, by use of seven adjustable parameters, The interaction with the 4ƒ5d<sup>2</sup> levels is discussed. A calculation of the levels of the 4ƒ<sup>2</sup>5d configuration was obtained with a rms error of ±182 cm<sup>-1</sup> relative to the 101 observed levels, utilizing 13 adjustable parameters.
RICHARD E. TREES, "4ƒ3 and 4ƒ25d Configurations of Doubly Ionized Praseodymium (Pr III)," J. Opt. Soc. Am. 54, 651-657 (1964)