2. Force fields

The length of the bond between a carbon atom and a hydrogen atom, whatever the molecule considered containing this group, is generally close to 1.1 Å (0.11 nm); similarly, the valence angle formed by hydrogen atoms, carbon and hydrogen, remains close to 109˚28', an angle found within the methane molecule. There are therefore "natural" values, i.e. free from any constraints, that can be transferred from one molecule to another; this is mainly true for organic compounds. The case of minerals is more complex.

This notion of the transferability of internal parameters was at the origin of the use of the first rigid models. In these models, atoms are balls with a number of holes dictated by their hybridization. These holes allow the right number of bonds to be made with other atoms. The main purpose of such models is to visualize the spatial arrangement of molecules....