United-atom model

In the united-atom model the $CH_{2}$ and $CH_{3}$ units are treated as one unified interaction site (hence united-atom), i.e. the hydrogen atoms are incorporated within the carbon atoms and are not expressed explicitly. Many force fields make use of this approximation in order to reduce the computational overheads. For example, n-hexane consists of 6 carbon atoms and 14 hydrogen atoms. A fully-atomistic model of n-hexane would require 20 interaction sites. Using the united-atom approximation, this reduces to two $CH_{3}$ sites and four $CH_{2}$ sites, a significant reduction.

References

1. Søren Toxvaerd "Molecular dynamics calculation of the equation of state of alkanes", Journal of Chemical Physics 93 pp. 4290-4295 (1990)