I am doing SMD of protein using NAMD with constant velocity pulling. In order to do the simulation, we have to specify the smd-atom (the atom to which the dummy atom is attached and then pulled). It is clearly given in the NAMD-tutorial. But, I want to pull a group of atoms, i.e., I need to get the centre of mass of the atoms and then attach the dummy atom to it. I am not sure how to add that in the input file where we give the force conditions and also mention the fixed atom.