Hybrid Quantum Mechanics/ Molecular Mechanics Methods and their Application

Abstract

Hybrid quantum mechanics (QM)/molecular mechanics (MM) methods allow simulations for much larger systems than accessible by QM methods alone. The size of many systems of topical interest in chemistry and biochemistry prevents efficient and accurate treatment by quantum mechanical ab initio methods. For reactions in condensed phase and surfaces periodic boundary conditions (PBC) can be applied reducing the size of the problem to a unit cell [1–3]. However, many interesting structure features such as defects or active sites require larger unit cells due to broken space and translation symmetry. A computationally appealing alternative are interatomic potential functions ranging from molecular mechanics force fields to ion-pair potentials. They yield accurate equilibrium structures for the type of systems for which they are parameterized [4], but are usually not suitable to describe the active sites of catalysts with sufficient accuracy. Moreover, unless special modifications are made, they cannot be used to model reactions in which chemical bonding is changed. The cluster model approach is an alternative that makes the calculations on active sites and defects feasible to ab initio methods [5]. Only a fragment of the structure is considered that contains the interesting part, and the surroundings are neglected or approximately included. There exist, however, classes of problems, which require a computational treatment of the whole system. A prominent example is shape selectivity in zeolite catalysis. Although zeolite catalysts with different framework structures have the same active sites in common, they may show very different catalytic performances.