Contact Search Algorithm

Hello,
I do not know the exact version that LIGGGHTS is using but the general approach can be found here.
https://en.wikipedia.org/wiki/Cell_lists

Basically, you place every cell into a bin(s) then when you get to atom i, you look at the bins around the bin that contains atom i.
This way you do not need to worry about every atom in the simulation and just those "near" the atom you want.

In LIGGGHTS it's typically a combination of cell lists and Verlet lists: https://en.wikipedia.org/wiki/Verlet_list
Implementation can be found in
https://github.com/CFDEMproject/LIGGGHTS-PUBLIC/blob/master/src/neighbor...
https://github.com/CFDEMproject/LIGGGHTS-PUBLIC/blob/master/src/neigh_gr...