Halogen-mediated exchange in the coupled-tetrahedra quantum spin systems Cu<SUB>2</SUB>Te<SUB>2</SUB>O<SUB>5</SUB>X<SUB>2</SUB>(X=Br,Cl)

Motivated by recent discussion on possible quantum critical behavior in the coupled Cu-tetrahedra system Cu2Te2O5Br2, we present a comparative ab initio study of the electronic properties of Cu2Te2O5Br2 and the isostructural Cu2Te2O5Cl2. A detailed investigation of the copper-copper interaction paths reveals that the halogen ions play an important role in the inter-tetrahedral couplings via X4 rings (X=Br,Cl). We find that, contrary to initial indications, both systems show a similar electronic behavior with long range exchange paths mediated by the X4 rings