In this paper, they separated knock-on damage (originating from the collision of the beam electrons with the nucleus of the target atom) from other radiation damage mechanisms (e.g., ionization damage or chemical etching) by the comparison of ordinary (12C) and heavy (13C) graphene because any chemical effect, or ionization damage, would not distinguish between 12C and 13C. Only knock-on damage would distinguish between them. They did it experimentally.
Knock-on damage can be modeled with molecular dynamics by assigning different initial velocities to an atom hit by an electron beam.
How to do molecular dynamics with different isotopes of the same element? Is there any DFT or MD code that does it?
I searched VASP wiki, and didn't find anything about isotopes.