Large - scale relativistic configuration - interaction calculation of the 2 s 2 1 S 0 – 2 s 2 p 3 P 1 intercombination transition in C III

A large-scale, relativistic configuration-interaction ~RCI! method has been developed for precision calculations of transition oscillator strengths. It is based on the no-pair Hamiltonian and employs finite B-spline basis functions. For the 2s S0 –2s2p P1 intercombination transition in berylliumlike carbon, the present RCI expansions reach close to 200 000 configurations, and include all single and double excitations from valencevalence, core-valence, and core-core interactions, along with dominant triple and quadruple excitations. Resulting lengthand velocity-gauge transition rates are very well converged, but still differ by a factor of 2. This strong gauge dependence is found to arise from the neglect of negative-energy states which has negligible effects on length-gauge results but can affect velocity-gauge results significantly. The present intercombination transition rate for C III of 101.660.7 sec differs from the measured value of 102.9460.14 sec @Doerfert et al., Phys. Rev. Lett. 78, 4355 ~1997!# by about 1.3%.