Thermodynamics Estimation Using Group AdditivityΒΆ
Benson’s Group Additivity approach, used by RMG, divides a molecule into functional groups, and the contribution of each functional group to the overall thermodynamics is included. RMG’s kinetic rules use a similar strategy: the reaction rate coefficient depends on the two (or more) functional groups involved in the reaction. The rate coefficient rule database uses an innovative tree structure that allows users to include ever-more specific defintions of the functional groups and the kinetics involving them. In both cases, users can easily modify and expand these databases to suit their needs. In principle, the trees could be expanded until all possible molecules and data for the reactions between all possible pairs of molecules have been included (of course, in practice, the database is much more limited). The kinetics database includes 19 reaction types (“families”), including hydrogen abstractions, cycloadditions, disproportionation reactions, radical recombination reactions, intramolecular rearrangements, etc.
Each reaction family includes data on many possible functional group pairings involved in the reaction, for a total of roughly 1000 rate rules. When a perfect match for a given reaction is not found in the database, RMG estimates the rate coefficient parameters based on “nearby” rate coefficient rules that are similar to the desired one. In addition to modifying the RMG databases, the user may also enter thermodynamic or kinetic information for specific molecules/reactions into Primary Thermo/Kinetic Libraries which supersede the estimated values. In this manner, precise data for well-studied reactions/species can be included.
