The problem of large-scale molecular dynamics simulations of iron has recently attracted attention in connection with the need to understand the microscopic picture of radiation damage in ferritic steels. In this paper we review the development of a new interatomic potential for magnetic iron, and describe the first large-scale atomistic simulations performed using the new method. We investigate the structure and thermally activated mobility of self-interstitial atom clusters and show that the spatial distribution of magnetic moments around a cluster is well correlated with the distribution of hydrostatic pressure, highlighting the significant part played by magneto-elasticity in the treatment of radiation damage. We show that self-interstitial atom clusters exhibit a transition from relatively immobile configurations containing (1 1 0)-like groups of atoms to (1 1 1)-like configurations occurring at a critical cluster size N-c similar to 5 atoms. We discuss implications of this finding for the treatment of cascade damage effects, and the possibility of observing new low-temperature resistivity recovery stages in neutron-irradiated alpha-iron. (C) 2006 Elsevier Ltd. All rights reserved.