Thanks for ur reply.
The airebo can be seen as a pair wise potential, it takes the form as Vij=V_Rij+bij*V_Aij, so Vij is the interaction between two atoms in the system, as long as the distance between these two is in the cutoff range. Of course when calcluate this we need to consider the other neighbor atoms, since the bij is a function of cos(ijk), this is a 3-body term. But this does not change the Vij is the pair energy of atom i and j. I believe the paire_airebo.cpp is calculating the totoal energy by summing all these pair_wise interactions up. I just wanna get a specific interaction between atom A and atom B out. Seems no easier way to find this out.