def output_system_info(filename, n_atoms, n_frames):
    """print system information"""
    print('System information:\n')
    print('        XYZ trajectory file name:\t{:s}'.format(filename))
    print('  Number of particles in the box:\t{:d}'.format(n_atoms))
    print('       Total number of snapshots:\t{:d}\n'.format(n_frames))
