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 , AM 1, and PM 3 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 , AM 1, and PM 3 methods. As a case in point, the draw‐back of AM 1 favoring bifurcated H‐bonded structures over single ones is not presented in the combined AM 1/ OPLS scheme. Possible ways of improvement of the combined QM / MM potential are discussed.