Atomic data from the IRON project XLII. Electron impact excitation of Fe XXI

Collision strengths and collisional rate parameters are provided for transi tions among the n = 2 and 2p3l levels of C-like Fe XXI. The data have been computed in a 52-state Breit-Pauli approximation thus accounting for the mo st prominent relativistic effects. An R-matrix close-coupling calculation c omprising partial wave contributions up to J = 59/2 and of order 8000 energ y points in the range up to 260 Ryd gave cross sections expected to be accu rate to better than 20% for transitions among the n = 2 levels.