United-atom model
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In the united-atom model the , and 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 sites and four sites, a significant reduction. The interaction sites are usually parametrised Lennard-Jones potentials.
Justification[edit]
Parameters[edit]
References[edit]
- Related reading
- Rolf Lustig and William A. Steele "On the thermodynamics of liquid propane", Molecular Physics 65 pp. 475-486 (1988)
- Sumnesh Gupta, Jia-an Yang and Neil R. Kestner "Computer modeling of liquid propane using three-site potential models", Journal of Chemical Physics 89 pp. 3733-3741 (1988)
- Søren Toxvaerd "Molecular dynamics calculation of the equation of state of liquid propane", Journal of Chemical Physics 91 pp. 3716-3720 (1989)
- Søren Toxvaerd "Molecular dynamics calculation of the equation of state of alkanes", Journal of Chemical Physics 93 pp. 4290-4295 (1990)
- Richard A. Messerly, Thomas A. Knotts IV, and W. Vincent Wilding "Uncertainty quantification and propagation of errors of the Lennard-Jones 12-6 parameters for n-alkanes", Journal of Chemical Physics 146 194110 (2017)