  We study the electronic properties of GaV4S8 (GVS) and GaTaSe8 (GTS), two
distant members within the large family of chalcogenides AM4X8, with A={Ga,
Ge}, M={V, Nb, Ta, Mo} and X={S, Se}. While all these compounds are Mott
insulators, their ground state show many types of magnetic order, with GVS
being ferromagnetic and GTS non-magnetic. Based on their bandstructures,
calculated with Density Functional Theory methods, we compute an effective
tight binding Hamiltonian in a localised Wannier basis set, for each one of the
two compounds. The localised orbitals provide a very accurate representation of
the bandstructure, with hopping amplitudes that rapidly decrease with distance.
We estimate the super-exchange interactions and show that the Coulomb repulsion
with the Hund's coupling may account the for the different ground states
observed in GVS and GTS. Our localised Wannier basis provides a starting point
for realistic Dynamical Mean Field Theory studies of strong correlation effects
in this family compounds.
