[gmx-users] potassium

[Matteo Masetti]

Dear gmx_users,
I'm aware that this question has been posted many times before, but I'm
still very confused.
I would like to perform a molecular dynamics symulation of a potassium
channel embedded in a lipidic environment, WITH potassium ions inside.
To match with the dppc topology and the lipid parameters supplied by
Tieleman, I choosed the newest ffgmx force-field (GROMOS-87 based ff). 
I've found in the mailing list that only GROMOS-96 and OPLS contain
potassium parameters, even if in ffgmxnb.itp the potassium LJ C6 and C12
parameters are present:    

K    39.10000       0.000       A   1.63787E-05   1.18384E-06

My question is: am I doing some terrible mistake?
Are these parameters enough for Gromacs to compute the Lennard-Jones
potential? Or I have to edit the .itp with each pairwise interaction,
say:

OW   K    1    Cij(6)   Cij(12)

Please forgive me if I'm missing any trivial point,
any help would be very useful.