We then run the collision for 20 time units, with the standard
N-body integrator based on the Barnes
& Hut "hierarchical tree"
algorithm^{5.4}:

7% hackcode1 in=i001.dat out=r001.dat tstop=20 freqout=2 \ freq=40 eps=0.05 tol=0.7 options=mass,phase,phi > r001.log

The integration frequency relates to the integration timestep as `freq` = , the softening length `eps` = , and
opening angle or tolerance `tol` = . A major output of
masses, positions and potentials of all particles is done every `1/freqout` = 0.5 time units, which corresponds to about 1/5 of a
crossing time. The standard output of the calculation is diverted to a
file `r001.log` for convenience. This is an (ASCII) listing,
containing useful statistics of the run, such as the efficiency of the
force calculation, conserved quantities etc. Some of this information
is also stored in diagnostic sets in the structured binary
output file `r001.dat`.

As an exercise, compare the output of the following two commands:

8% more r001.log 9% tsf r001.dat | more

(c) Peter Teuben