A COMBINED QUANTUM CHEMICAL MOLECULAR MECHANICAL STUDY OF HYDROGEN-BONDED SYSTEMS

A computational approach, which involves the combination of the OPLS force field and molecular orbital MNDO, AM1, and PM3 methods, has been developed to describe the effects of a large, molecular mechanically simulated environment on the Hamiltonian of a quantum chemical system. To test the validity of the combined quantum mechanical/molecular mechanical (QM/MM) potential, a systematic study of the structures and energies of neutral and charged hydrogen-bonded complexes has been carried out, including comparisons with pure semiempirical calculations and available experimental and ab initio data. It is shown that, in many cases, the hybrid QM/MM potential behaves better than do related MNDO/M, AM1, and PM3 methods. As a case in point, the drawback Of AM1 favoring bifurcated H-bonded structures over single ones is not presented in the combined AM1/OPLS scheme. Possible ways of improvement of the combined QM/MM potential are discussed.