How to use the amuse.units.nbody_system function in amuse

from amuse.ext.bridge import bridge
from amuse.community.hermite0.interface import Hermite
from amuse.community.agama.interface import Agama
from matplotlib import pyplot
from amuse.ic.kingmodel import new_king_model

if __name__ in ('__main__', '__plot__'):

    # set up parameters:
    N = 100
    W0 = 3
    Rinit = 50. | units.parsec
    timestep = 0.01 | units.Myr
    Mcluster = 4.e4 | units.MSun
    Rcluster = 0.7 | units.parsec
    converter = nbody_system.nbody_to_si(Mcluster,Rcluster)

    # create a globular cluster model
    particles = new_king_model(N, W0, convert_nbody=converter)
    particles.radius = 0.0| units.parsec
    cluster = Hermite(converter, parameters=[("epsilon_squared", (0.01 | units.parsec)**2)], channel_type='sockets')

    # create the external potential of the Galaxy
    galaxy = Agama(converter, type="Dehnen", gamma=1.8, \
        rscale=1000.| units.parsec, mass=1.6e10 | units.MSun, channel_type='sockets')

    # shift the cluster to an orbit around Galactic center
    acc,_,_ = galaxy.get_gravity_at_point(0|units.kpc, Rinit, 0|units.kpc, 0|units.kpc)
    vcirc = (-acc * Rinit)**0.5
    print("Vcirc=%f km/s" % vcirc.value_in(units.kms))
    particles.x  += Rinit
    particles.vy += vcirc