Purple acid phosphatases (PAPs) constitute a new class of metalloenzymes that catalyze the hydrolysis of certain phosphate esters, including nucleoside di- and triphosphates and aryl phosphates, under acidic conditions. To provide some insight into these metalloenzymes we have performed quantum chemical and molecular mechanics calculations based on the mixed-valence [Fe^IIFe^III(BPBPMP)(OAc)₂]⁺model complex (1) (H₂BPBPMP = 2-bis[{(2-pyridylmethyl)-aminomethyl}-6-{(2-hydroxybenzyl)-(2-pyridylmethyl)}-aminomethyl]-4-methylphenol). The geometric and the vibrational parameters calculated by molecular mechanics show that the force fields established in this work reproduce the binuclear iron core with µ-phenoxo or µ-alkoxo and di-µ-acetate bridges presented in the PAPs model complexes. The atomic orbital analysis of the SOMO contributions indicated that the Fe^IIIatom and the terminal phenolate are involved in the phenolate to Fe^IIIcharge transfer electronic transition in 1 as argued from electronic spectroscopic data in the PAPs. Key words: mixed-valence Fe^IIFe^IIIcomplex, purple acid phosphatases, molecular modeling.