[gmx-users] mixing rules for LJ parameters?

[Mark Abraham]

Nicolas Winter wrote:
> Hi,
> 
> 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.

With what value are you comparing it - and what is it?

Does this arise for any other atom pairs?

Mark