quantummechanicsmolecularmechanics

Quantum mechanics molecular mechanics, often abbreviated as QM/MM, is a hybrid computational method used to study molecular systems. It combines the accuracy of quantum mechanical (QM) methods for describing the electronic structure of a small, critical region of a system with the computational efficiency of molecular mechanics (MM) for the surrounding environment. This approach is particularly useful for studying large or complex molecules where a full QM treatment would be prohibitively expensive.

In a typical QM/MM calculation, a system is divided into two parts: the QM region and the MM region. The QM region is treated with a high-level quantum chemical method, such as density functional theory (DFT) or Hartree-Fock, to accurately capture electronic effects like bond breaking and formation, charge transfer, and excited states. The MM region, which constitutes the majority of the system, is treated using classical mechanics with a force field. This force field describes interactions between atoms using simple potential energy functions based on bonded and non-bonded terms.

The QM and MM regions are coupled together, allowing for the transfer of information between them. This coupling is crucial for accurately describing the interactions between the reactive QM part and its environment. QM/MM methods are widely applied in various fields, including biochemistry (e.g., enzyme catalysis, protein-ligand binding), materials science, and drug discovery. They provide a computationally feasible way to explore complex chemical processes that are beyond the reach of pure QM or MM methods alone. The accuracy of QM/MM results depends on the choice of QM method, MM force field, and the way the QM/MM interface is treated.