Generalized gradient approximation cluster calculations

Calculations have been made in the framework of the Generalized Gradient Approximation (GGA) by means of the Becke-Perdew functional [22,23]. It is worth mentioning, however, that in the case of Cr in fluorides the Re values computed in the Local Density Approximation (LDA) for a 21 atoms cluster differ only by about 1% from those obtained through GGA [20]. The effects of the electrostatic potential due to the rest of the ions not included in the IFeAeMe " cluster have been considered in all the calculations. When such a potential is not taken into account the computed distances increase by about 1%. A similar situation is found in the case of Cr in elpasolites [12] where a further analysis on this point is given. 

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