Publisher Summary Modern NMR techniques can give some indications about interatomic distances for liquid samples, geometrical information, for large or medium-size molecules, is up to now mainly derived from crystallographic data. An important role is played in the field by techniques known as molecular mechanics (MM) or empirical force field methods. The high precision of MM is largely supported by comparison with other (quantum) approaches as well as experimental data (X-ray crystallography). This type of method should continue to play an essential role in structural analysis owing to the increasing interest devoted to large biomolecules for which it is necessary to have a calculation method that is fast and easy to implement at one's disposal. For complex systems, this set of primitives should allow a person to quickly guess more realistic geometries to start the optimization processes. Basically, molecular mechanics treats molecules as being composed of masses and springs. It uses the laws of classical mechanics to treat the diverse interactions occurring in the real molecule, according to a model that is empirically parameterized. Molecular mechanics offers extended capabilities as to the treatment of non-standard situations that cannot be represented easily with such naive models. Molecular mechanics also gives important information about energetics such as more stable or low-energy conformers and interconversion pathways.