Generalized gradient approximation Becke

The calculations are performed within the framework of the Carr-Parrinello molecular dynamics [25, 26]. Our approach is based on the density functional theory with generalized gradient approximations after Becke for the exchange energy [27] and Lee, Yang, and Parr for the correlation energy [28]. Troullier-Martins type... 

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. 

We adopted the method by Car and Parrinello [22] to ensure a self-consistent evolution of the electronic structure during molecular dynamics motion. The electronic structure was described in the framework of density functional theory (DFT) with the generalized gradient approximation (GGA) due to Becke (B) for the exchange energy and Lee, Yang and Parr (LYP) for the correlation energy... 

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