where E(Na x FeFe(CN) 6 ) represent the total energy per f.u. for a system with x intercalated sodium atoms. E(Na 2 FeFe(CN) 6 ) and E(FeFe(CN) 6 ) are the reference energies per f.u. for the completely sodiated and desodiated systems, respectively. The molar fraction of sodium intercalated in the material is represented by x 2 .