  Recent gravitational wave observations of neutron star-neutron star and
neutron star-black hole binaries appear to indicate that massive neutron stars
may not be too uncommon in merging systems. In this manuscript, we present a
first set of evolution of massive neutron star binaries using Monte-Carlo
radiation transport for the evolution of neutrinos. We study a range of
systems, from nearly symmetric binaries that collapse to a black hole before
forming a disk or ejecting material, to more asymmetric binaries in which tidal
disruption of the lower mass star leads to the production of more interesting
post-merger remnants. For the latter type of systems, we additionally study the
impact of viscosity on the properties of the outflows, and compare our results
to two recent simulations of identical binaries performed with the WhiskyTHC
code. We find agreement on the black hole properties, disk mass, and mass and
velocity of the outflows within expected numerical uncertainties, and some
minor but noticeable differences in the evolution of the electron fraction when
using a subgrid viscosity model, with viscosity playing a more minor role in
our simulations. The method used to account for r-process heating in the
determination of the outflow properties appears to have a larger impact on our
result than those differences between numerical codes. We also use the
simulation with the most ejected material to verify that our newly implemented
Lagrangian tracers provide a reasonable sampling of the matter outflows as they
leave the computational grid. We note that, given the lack of production of hot
outflows in these mergers, the main role of neutrinos in these systems is to
set the composition of the post-merger remnant. One of the main potential use
of our simulations is thus as improved initial conditions for longer evolutions
of such remnants.
