Combined quantum mechanism and molecular mechanism (QM/MM) has gained increased use in the study of chemical reactions in solution and in enzymes. In this method, a large molecular system is partitioned into a quantum mechanical region and a molecular mechanical region, such that only a small part of the system is treated explicitly by quantum mechanics, but still retains the effects of the surrounding environment. Consequently, combined QM/MM methods take advantage of the accuracy of quantum mechanics and computational efficiency of molecular mechanics. In this paper, we describe recent progresses in the development of novel combined QM/MM techniques, and summarize results of several applications to condensed phase chemistry and enzymology.

Key words: QUANTUM MECHANICS, MOLECULAR MECHANICS, MOLECULAR ORBITAL THEORY, OROTIC ACID, NUCLEOSIDE, PHOSPHATE, DECARBOXYLASE