Dear Abdellatif,

There's no simple rule, the choice of proper theoretical model (functional + basis set) depends on system, phase, methodology, properties of interest and more...So, to find a suitable model, I strongly recommend that you follow literature, take some functionals and basis sets that seem appropriate for your system, then test them to obtain your own reliable combination.

In your case, use of simple split-valence (6-31G(d)...) or correlation-consistent (cc-pVDZ...) basis sets for main atoms (C, H, N, O...) , in conjunction with Effective core potentials ECPs (SDD, LANL2DZ ...) for Iron (Fe) atoms is probably a suitable model commonly employing for similar metal complexes.

Regards,

Dear Malek,

Thank you so much for your usefull answer!