[gmx-users] force application group wise

[Rana Ali]

Dear users

We were running MD system consists of ion and water through a nanotube
under a pressure gradient.
For that we are applying pressure in terms of force to the molecules.
Could any body help how to create group so that the external force will be
applied only on water molecules not on the ions. means how to define the
force in the .mdp file

Thanks in advance

Warm regards