The potential activities for two series of iron complex precatalysts on ethylene reactivity are evaluated by the density functional theory (DFT)–charge equilibration (DFT–QEq) method on the basis of the molecular mechanics (MM–QEq) method. All three spin states on the central iron atom are calculated through the two charge methods to obtain the effective net charge, in addition to the QEq method alone. The results show that reasonable charge values are obtained by the QEq method, well reflecting the influence of substituents and revealing correlations between the activity and the effective net charge. All these series of iron complex precatalysts present the same tendency: the potential activities are elevated along with a decreasing effective net charge on the iron atom. Relying on the electronic influence of substituents on the ligands, the variation trends are grouped into two classes, which present the same correlation.