Insight into sbmask()
Bitmask is a widely used technique to validate authority, often using the bitwise AND operation for fast operations. A well-known example is the subnet mask, which operates on IP addresses in a network and yields the routing prefix. In the context of bonded atoms in LAMMPS, sbmask() uses the same mechanism to identify whether a particle is linked by a special bond.
In order to analyze bitmask operations in the LAMMPS source code, we now return to the example LAMMPS script (test.in) and pair_lj_cut.cpp implemented as pair style lj/cut. As described in Chapter 4, Accessing Information by Variables, Arrays, and Methods, the ilist array represents a collection of central atoms to loop over, and the jlist array represents the corresponding neighbors of each member of ilist. Recall that in our example LAMMPS script, the cutoff radius is 5.5 units, so the first atom (i=0) has five pairwise atoms (j) interacting with it.
A short simulation run is performed and using...
