Principles
A group contribution method uses the principle that some simple aspects of the structures of chemical components are always the same in many different molecules. The smallest common constituents are the atoms and the bonds. The vast majority of organic components, for example, are built of carbon, hydrogen, oxygen, nitrogen, halogens, and maybe sulfur or phosphorus. Together with a single, a double, and a triple bond there are only ten atom types (not including astatine) and three bond types to build thousands of components. The next slightly more complex building blocks of components are functional groups which are themselves built of few atoms and bonds.
A group contribution method is used to predict properties of pure components and mixtures by using group or atom properties. This reduces the number of needed data dramatically. Instead of needing to know the properties of thousands or millions of compounds, only data for a few dozens or hundreds of groups have to be known.
Read more about this topic: Group Contribution Method
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