  We present a microscopic study of the electronic and magnetic properties of
the newly synthesized Cu(II)-spin tetrahedron-based Cu4Te5O12Cl4 compound based
on Density Functional calculations and on ab initio-derived effective models.
In view of these results, we discuss the origin of the observed differences in
behavior between this system and the structurally similar and much studied
Cu2Te2O5Cl2. Since the Br analog of the title compound has not been synthesized
yet, we derive the crystal structure of Cu4Te5O12Br4 by geometry optimization
in an ab initio molecular dynamics calculation and investigate the effect of
substituting Cl by Br as well as the possible magnetic behavior of this system
and compare with the recently studied sister compound, Cu2Te2O5Br2.
