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 [FeIIFeIII(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 FeIII atom and the terminal phenolate are involved in the phenolate to FeIII charge transfer electronic transition in 1 as argued from electronic spectroscopic data in the PAPs. Key words: mixed-valence FeIIFeIII complex, purple acid phosphatases, molecular modeling.