Primary knock-on atom velocity

I wanted to simulate the damge cascade in my sample (ZnO) using 20 MeV Fe atom…How to write the Pka velocity and what’s the unit of the pka velocity as it is not clearly mentioned in the manual…

velocity units depend on your units setting. they are - as to be expected - length unit / time unit.
please also note that several commands have a “units” keyword. that modifies the meaning of the length unit. with units box, it is the actual length unit, with units lattice (usually the default), it is length unit times lattice spacing in that direction.