Effective collision strengths for transitions within the ground configuration of Fe XXI

Effective collision strengths for transitions among the energetically lowes t 46 fine-structure levels belonging to the (1s(2)) 2S(2) 2p(2), 2s2p(3), 2 p(4), 2S(2)2p3s, 2s(2)2p3p and 2S(2)2p3d configurations of Fe XXI are compu ted, over an electron temperature range of 5.6 less than or equal to log T- e less than or equal to 7.4 K, using the recent Dirac Atomic R-matrix Code (DARC) of Norrington and Grant. Results are presented for transitions withi n the ground configuration only, and are compared with earlier R matrix cal culations. Large discrepancies are observed for many transitions, especiall y at lower temperatures.