Molecular mechanics can model very large compounds quickly.
Quantum mechanics is able to compute many properties and model chemical reactions. It is possible to combine these two methods into one calculation, which models a very large compound using molecular mechanics and one crucial section of the molecule with quantum mechanics. This is designed to give results that have very good speed where only one region needs to be modeled quantum mechanically. This can also be used to model a molecule surrounded by solvent molecules. The earliest combined calculations were done simply by modeling different parts of the system with different techniques.