[gmx-users] How do Charge Groups work?

[X.Periole]

> I had a system where I have no charges in it, but I do 
>have Lennard-Jones non-bonded interactions. I've noticed 
>that I get different values for the Lennard-jones 
>interaction energy differs depending how I split the 
>system up into charge groups (as the charge group concept 
>is kinda abitrary in a system with no charges), this can 
>not be right surely? 
> 
> As I understood it the charge groups were used to help 
>compile the neighbour lists, but what I want is all the 
>atoms within my cut-off radius to be in my pair-list (for 
>that atom). Is there an option to ignore the charge 
>groups? I find I get bigger Lennard-Jones interactions if 
>I put my whole system into a charge group that if I split 
>it up, but I need to split it up for my larger systems. 
>Is there a way to increase the maximum number of charge 
>groups? And is this a good idea?

You should check the manual for the option allowing a atom
base cutoff, I am not sure it actually exist.
A crude way to fix it would be to define each atom as a
seprate charge group.

XAvier