how to set variable timestep in the simulation

Dear lammps users:
Now I am do the gallium bombardent silicon,which the gallium ion has the velocity of 0.744 Angstrom/fs.And the ions impact the silicon one by one .During the cascades in silicon
atoms, the velocity of Ga ions greatly changes from 0.744 A/fs(when the ion energy equal to 2 keV) to less than 0.02 A/fs.Now I have to set the timestep.Taking into account the computational limitations and simulation accuracy, I want to set the MD time-step in a range of 0.002–1 fs during calculation, making sure no atom moves further than 0.02 Angstrom per iteration.
But I do not know how to write the command.Can anyone tell me how to realize it?Best regards.
Yang Yuanrui

Dear lammps users:
    Now I am do the gallium bombardent silicon,which the gallium ion has
the velocity of 0.744 Angstrom/fs.And the ions impact the silicon one by
one .During the cascades in silicon
atoms, the velocity of Ga ions greatly changes from 0.744 A/fs(when the
ion energy equal to 2 keV) to less than 0.02 A/fs.Now I have to set the
timestep.Taking into account the computational limitations and simulation accuracy,
I want to set the MD time-step in a range of 0.002–1 fs during
calculation, making sure no atom moves further than 0.02 Angstrom per
iteration.
But I do not know how to write the command.Can anyone tell me how to
realize it?Best regards.

​have you tried reading the manual?
http://lammps.sandia.gov/doc/fix_dt_reset.html​

​axel.​