The theoretical central-field-approximation predictions for the 3P and 1P terms of the 1s2 2s3p configuration in the N= 4 electron isoelectronic sequence are in violent disagreement with observation. Correlation effects in the form of configuration interaction have been included by considering the mixing among nine odd configurations. Even though the terms are displaced considerably, they remain quite pure, and after configuration mixing the agreement between theory and observation is good. In order to show the use of such theoretical calculation, weighted oscillator strengths for the 2s21S1-2s3p1P1, 3P1 transitions are also computed.
HALIS ODABASI, "Configuration Mixing in the N=4 Electron Isoelectronic Sequence," J. Opt. Soc. Am. 59, 583-588 (1969)