In this paper we Study kink defects on the single period (SP) reconstructed 90 degrees partial dislocation in silicon. First, we use the Tersoff potential in order to determine how to set up the supercells we are employing so that we can extract meaningful results. In particular, we consider the elastic interactions between all dislocation segments and their effect on the energetics of the cells. We have identified a new long-range elastic field associated with the SP structure and we model it as arising from a dipolar line of force along the dislocation line. We also Study the effect of this field on kink energies. With this information we then perform first principles simulations. Our results indicate that the only stable kink defects are those that separate regions of the dislocation with opposite reconstruction senses. We study the Structure of these defects or complexes, Lis they are called, and we obtain the formation energy of a complex pair. We also investigate the changes in electronic structure and the rebonding that take place during the motion of these defects. We obtain the energy of migration of a single complex. Using a tight binding approach, we obtain the bandstructures associated with the complexes both at stable positions and at the migration saddle point and we discuss their role in the dislocation dopant effect. (C) 2006 Elsevier Ltd. All rights reserved.