[gmx-users] mixing rules for LJ parameters?

Nicolas Winter n-winter at northwestern.edu
Mon Dec 18 22:48:31 CET 2006


I'm trying to understand how the LJ parameters are determined in the 
Gromacs FF, namely the file ffgmxnb.itp. According to the mixing rules 
given on page 88 of the manual c6 and c12 for interactions between atoms 
should just be the square root of the product of the individual atom 
parameters. However when I tried to take the individual c6 and c12 for O 
and N for example, and mix them according to this rule I come up with 
the correct number for for the O - N c6 value, but /not /for c12. So I'm 
curious how the c12 is determined? Here are the numbers I'm working 
with, the individual atom parameters taken directly from the Gromacs FF, 
file ffgmxnb.tip. The O-N values are what I get from using the mixing 
rule combination 1 on p88.

Any insights would be greatly appreciated!

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