N = timestep number
reset_timestep 0 reset_timestep 4000000
Set the timestep counter to the specified value. This command normally comes after the timestep has been set by reading a restart file via the read_restart command, or a previous simulation advanced the timestep.
This command cannot be used when any fixes are defined that keep track of elapsed time to perform certain kinds of time-dependent operations. Examples are the fix deposit and fix dt/reset commands. The former adds atoms on specific timesteps. The latter keeps track of accumulated time.
Various fixes use the current timestep to calculate related quantities. If the timestep is reset, this may produce unexpected behavior, but LAMMPS allows the fixes to be defined even if the timestep is reset. For example, commands which thermostat the system, e.g. fix nvt, allow you to specify a target temperature which ramps from Tstart to Tstop which may persist over several runs. If you change the timestep, you may induce an instantaneous change in the target temperature.
Resetting the timestep clears flags for computes that may have calculated some quantity from a previous run. This means these quantity cannot be accessed by a variable in between runs until a new run is performed. See the variable command for more details.