Charge transfer in biologically important molecules: Comparison of high-level ab initio and semiempirical methods

Recent observations in both experimental and theoretical chemistry indicate the importance of charge transfer on the energetics and charge distributio n of biomolecular systems. Here we present a detailed analysis of hydrogen- bonded complexes to assess the quality of Hartree-Fock (HF) and semiempiric al methods in predicting the amount of charge transferred. We find that bot h HF and semiempirical methods systematically overestimate charge transfer when compared to MP2/6-311++G** calculations. Although inclusion of electro n correlation is important for a proper assessment of charge transfer, both HF/6-311++G** and AM1 can serve as good approximations to charge transfer in biomolecular systems. (C) 2000 John Wiley & Sons, Inc.