  We use a set of numerical N-body simulations to study the large-scale
behavior of the reduced bispectrum of dark matter and compare the results with
the second-order perturbation theory and the halo models for different halo
mass functions. We find that the second-order perturbation theory (PT2) agrees
with the simulations fairly well on large scales of k<0.05 h/Mpc, but it shows
a signature of deviation as the scale goes down. Even on the largest scale
where the bispectrum can be measured reasonably well in our simulations, the
inconsistency between PT2 and the simulations appears for the colinear triangle
shapes. For the halo model, we find that it can only serve as a qualitative
method to help study the behavior of Q on large scales and also on relatively
small scales. The failure of second-order perturbation theory will also affect
the precise determination of the halo models, since they are connected through
the 3-halo term in the halo model. The 2-halo term has too much contribution on
the large scales, which is the main reason for the halo model to overpredict
the bispectrum on the large scales. Since neither of the models can provide a
satisfying description for the bispectrum on scales of about 0.1h/Mpc for the
requirement of precision cosmology, we release the reduced bispectrum of dark
matter on a large range of scales for future analytical modeling of the
bispectrum.
