We report here the comparison of several GGA and hybrid-type functionals in combination with different basis sets to describe the precursor catalysts of the iron bis(arylimino)pyridine family that can be activated with methylaluminoxane to oligomerize or polymerize ethylene.From our calculations, it follows that hybrid functionals like B3LYP better take into account magnetic (spin) properties as compared to BP86 or BLYP.In addition, a quantitative structure activity relationship (QSAR) has been built comprising in total 10 different precursors. From this correlation it follows that the position of the aryl groups in the bis(arylimino)pyridyl complexes plays an important role with respect to the activity of the catalysts.