process by which two or more molecules
combine and rearrange to form a new molecule
or series of molecules.
Reactions are the basis of most chemical processes and understanding them the
essence of much of chemistry.
Reactions usually involve the making and breaking of bonds, thus their accurate modeling requires quantummechanical
process that transforms one or more chemical species
into others. Typical reactions make or break bonds;
others change the state
of ionization or other properties taken to distinguish chemical species.