  Interactions such as mergers and flybys play a fundamental role in shaping
galaxy morphology. Using the Horizon Run 4 cosmological N-body simulation, we
studied the frequency and type of halo interactions, and their redshift
evolution as a function of the environment defined by the large-scale density,
pair separation, mass ratio, and target halo mass. Most interactions happen at
large-scale density contrast $\delta \approx 20$, regardless of the redshift,
corresponding to groups and relatively dense part of filaments. However, the
fraction of interacting target is maximum at $\delta \approx 1000$. We provide
a new empirical fitting form for the interaction rate as a function of the halo
mass, large-scale density, and redshift. We also report the existence of two
modes of interactions from the distributions of mass ratio and relative
distance, implying two different physical origins of the interaction. Satellite
targets lose their mass as they proceed deeper into the host halo. The relative
importance of these two trends strongly depends on the large-scale density,
target mass, and redshift.
