Abstract
An R-matrix calculation for the electron impact excitation of the beryllium-like ion C III is described, in which the first twelve states formed from the 2s2, 2s2p, 2p2 and 2s3l configurations are included in the expansion of the total wavefunction. Effective collision strengths for electron temperatures between 12000 and 50000K are calculated for all transitions between the lowest six states, and are compared with those obtained in an earlier six-state R-matrix calculation. In contrast to earlier work on the beryllium-like ion O V, the effective collision strength for 2s2 1S-2s2p 3P0 in C III is not significantly affected by resonances converging to the 2s3l thresholds. However, some other dipole-forbidden transitions do show significant differences, particularly at the lowest temperatures. The biggest effect is for 2s2 1S-2p2 1S, where the twelve-state effective collision strength is double the six-state one at all temperatures considered.