[gmx-users] units for Leonard-Jones parameters in gromacs topology file

Mark Abraham Mark.Abraham at anu.edu.au
Thu Jul 6 07:25:12 CEST 2006

Vanessa Oklejas wrote:
> Hi All,
> A simple question: What are the units for the Leonard-Jones potential 
> parameters "r"and "epsilon" in the gromacs topology files.

r is the variable, sigma and epsilon are the parameters in equation 4.5 
of the manual.

In that equation the ratio of sigma and r is raised to two different 
powers and the results are added together, so that ratio must be 
dimensionless, thus sigma must have dimensions of length to match r. 
Since V_LJ has dimensions of energy, so too must epsilon. See Tables 2.1 
and 2.2 for the units for these dimensions.

...or you could look in section 5.3.3 of the manual where they are 
defined explicitly.

Please have a look in the manual before asking the sort of question the 
manual should answer - you'll learn better that way!


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