I am trying to set the velocity for a group so that is does not vary for the course of a simulation. This is analogous to the freeze command with respect to atom position. I know of no way to do this in the existing framework of the .mdp file options. I do not have much experience coding but I think it should be pretty easy to write this option into the grompp file, though perhaps I am being naive. I am asking if you have a simple list of files and locations where I need to change code, should I be altering the grompp file or is there some way to do it using user defined options? I figure I can set the force on the group equal to zero at each time step, or set the velocity equal to the specified velocity after each time step. This latter way may avoid screwing up pressure calculations and such. Thanks, Jon P.