The program generates various hydrogen-disordered ice structures without defects. The total dipole moment is set to zero unless you change the polarization behavior with --pol_loop_1, --pol_loop_2, or --target_polarization. The minimal structure (with --rep 1 1 1) is not always a single unit cell, because handling the hydrogen-bond network topology of very small lattices under periodic boundary conditions is difficult. Note that the generated structure is not optimized for potential energy.
To generate a large supercell of ice Ih in CIF format:
genice3 1h --rep 8 8 8 -e cif > 1hx888.cif
To generate an ice V lattice with a different hydrogen order in CIF format, use the -s option to set the random seed:
genice3 5 -s 1024 -e cif > 5-1024.cif
To generate an ice VI lattice at a different density with the TIP4P water model in GROMACS format:
genice3 6 --density 1.00 -e "gromacs :water_model tip4p" > 6d1.00.gro
GenIce3 is modular: it loads unit cells from plugins in the unitcell folder, places water and guest molecules using plugins in the molecules folder, and writes output via plugins in the exporter folder. You can add your own plugins to extend GenIce3; many plugins accept options.