Difference between revisions of "United-atom model"

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In the '''united-atom model''' the <math>CH_2</math> and <math>CH_3</math> units
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{{Stub-general}}
are treated as one point, i.e. the hydrogen atoms are incorporated within  
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In the '''united-atom model''' the <math>CH_2</math>,  <math>CH_3</math> and <math>CH_4</math> units
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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]]
 
the carbon atoms and are not expressed explicitly. Many [[force fields]]
 
make use of this approximation in order to reduce the computational  
 
make use of this approximation in order to reduce the computational  
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14 hydrogen atoms. A fully-atomistic model of ''n''-hexane would  
 
14 hydrogen atoms. A fully-atomistic model of ''n''-hexane would  
 
require 20 interaction sites. Using the united-atom approximation, this reduces to  
 
require 20 interaction sites. Using the united-atom approximation, this reduces to  
two <math>CH_3</math> sites and four <math>CH_2</math> sites, a significant reduction.
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two <math>CH_3</math> sites and four <math>CH_2</math> sites, a significant reduction. The interaction sites are usually parametrised [[Lennard-Jones model | Lennard-Jones potentials]].
 
==Justification==
 
==Justification==
 
==Parameters==
 
==Parameters==
 
==References==
 
==References==
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<references/>
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;Related reading
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*[http://dx.doi.org/10.1080/00268978800101181 Rolf Lustig and William A. Steele "On the thermodynamics of liquid propane", Molecular Physics '''65''' pp. 475-486 (1988)]
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*[http://dx.doi.org/10.1063/1.454894 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)]
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*[http://dx.doi.org/10.1063/1.456853 Søren Toxvaerd "Molecular dynamics calculation of the equation of state of liquid propane", Journal of Chemical Physics '''91''' pp. 3716-3720 (1989)]
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*[http://dx.doi.org/10.1063/1.458709 Søren Toxvaerd "Molecular dynamics calculation of the equation of state of alkanes", Journal of Chemical Physics '''93''' pp. 4290-4295 (1990)]
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*[http://dx.doi.org/10.1063/1.4983406 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)]
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[[Category: Models]]
 
[[Category: Models]]

Latest revision as of 12:21, 23 May 2017

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In the united-atom model the CH_2, CH_3 and CH_4 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. The interaction sites are usually parametrised Lennard-Jones potentials.

Justification[edit]

Parameters[edit]

References[edit]

Related reading