Effect of halogen displacements on magnetic coupling for methyloxo-bridged copper(II) dimers is investigated by means of DFT-BS (broken-symmetry) approach. Both coupling constants J and bridging 0 atomic charges(Q(O)) increase with the increasing displacement degree n. The displacements lead to beta electron transfer from Cl to Cu atoms, alpha electron from O atoms to methyl groups, which brings down spin densities on Cu2O2 core and further spread them toward the periphery of the models. The resulted spin delocalization reduces antiferromagnetic coupling strength for methyloxo-bridged copper (II) dimers. Furthermore, the decrease of spin densities on O p(x), O p(y) and Cu d(xy) orbitals is the leading reason for the reduction of coupling strength as halogen displacements. (C) 2003 Elsevier Science B.V. All rights reserved.