I could not find how to do this in SPH_LAMMPS_userguide.pdf.
Could you point me to some documentation please ?
decreasing the number of particles (this is a mesoscale model, so they
are not atoms) would require either creating a new data file or
deleting particles (e.g. through defining a region and using the
delete_atoms command). the data file format is described in the
documentation for the read_data command.
increasing the number of particles can be done via the replicate
command. however, only the z-direction is periodic, so you can only
use replication in z-direction. in that case, you probably want to
check out the settings of the processors command and also perhaps
consider enabling load balancing for optimal performance.