Collision energy in the ball mill

Hi.

I have a question

I want to calculate the collision energy between balls in a ball-mill.
So I'm trying get the value of the kinetic energy of balls when a ball contacts another ball.

The condition of atom type is granular,

I have used "compute pair/gran/local command ",

as describe below

compute pa all pair/gran/local id

and, I've gotten the IDs of balls(atoms) which contacts another balls with as c_pa[1], and c_pa[2].

I want to make group which have the atom IDs in c_pa[1], and c_pa[2] ,but i couldn't that.

I 've tried

group group-ID id c_pa[1]

didn't work.

I think I have mistaken fundamentally , but I could't specify about that.

I hope someone give some advises .