print out the force acting on the center of mass of a molecule


I want to a run a simulation in lammps with several polymers and I want to print out the positions of the COM and the force acting on the COM.
To print out the positions in the COM I know that I have to do:

compute CM all com/molecule
fix aveCM all ave/time 4000 1 4000 c_CM[1] c_CM[2] c_CM[3] file out_com.dat mode vector

how can I do it for the force?
I looked around in the manual and mail list but didn't find anything...
Thank you very much for your help!