The underlying idea of any Group Contribution method is as follows: whereas there are thousands of chemical compounds of interest to science and technology, the number of structural and functional groups which constitute all these compounds is very much smaller. The basic assumption is made that the physical property of a material (gas, liquid or solid) is a sum of contributions from each of the material's component parts. The fundamental assumption is additivity of these contributions.
The development and use of Group Contribution methods proceeds in two stages:
1) The properties of known materials are correlated with their chemical structure, in order to identify the basic groups and their Additive Molar Quantities (AMQ's), in the nomenclature of van Krevelen.
2) The properties of unknown materials are estimated through direct addition of AMQ's from constituent chemical groups, or through the use of additive quantities to estimate parameters in more accurate correlations.
These methods are largely empirical. In some cases, theoretical knowledge about the interdependence of material properties may be used as a guide in developing correlations.
Nevertheless, the definition of constituent "groups" is a very subjective matter. At one extreme, one may assume that only the basic atoms need be distinguished. However, we know that carbon in diamond exhibits very different properties from carbon in graphite. Even further, a carbonyl in a ketone is likely to exhibit different properties from the carbonyl in an organic acid. However, experience suggests that the carbonyl in most ketones, at least, are similar. Of course, the accuracy of any group contribution method increases as more and more distinctions are made between groups, until ultimately every compound comprises its own "group". The utility in the method comes from the wise selection of groups such that the number of groups remains small, but the accuracy of property estimation is still acceptable.
The correlations described under THEORY come from several sources. For further information on these, how they were derived and limitations on their accuracy, see:
Fedors, R.F., Poly. Eng. Sci, 14: 147,153 (1974).
Reid, R.C., Prausnitz, J.M. and Sherwood, T.K., The Properties of Gases and Liquids, 3rd Ed., McGraw-Hill: New York, 1977.
Van Krevelen, D.W., Properties of Polymers, 3rd Ed., Elsevier: Amsterdam, 1990.