Inquiry about nb3b/harmonic pair style


I am trying to understand how the nb3b/harmonic pair style works. The manual is a bit confusing for me. I what to make sure that I understood the following part correctly:

"Each entry has six arguments. The first three are atom types as referenced in the LAMMPS input file. The first argument specifies the central atom. The fourth argument indicates the K parameter. The fifth argument indicates theta_0. The sixth argument indicates a separation cutoff in Angstroms.

For a given entry, if the second and third arguments are identical, then the entry is for a cutoff for the distance between types 1 and 2 (values for K and theta_0 are irrelevant in this case).

For a given entry, if the first three arguments are all different, then the entry is for the K and theta_0 parameters (the cutoff in this case is irrelevant)."

According to this description, if I add the following line into the .nb3b.harmonic file for the B-A-B three body interaction, LAMMPS will only read the cutoff distance but not K or theta_0:

A B B k theta_0 cutoff

Since the first entry is always the central atom it seems it is not possible to define three body parameters for the B-A-B angle! Why it is necessary for the first three entries to be different? The force field which I use has a three body interaction defined for S-Zn-S angles. So, the first entries in my .nb3b.harmonic file would be “Zn S S”, but my understanding is that LAMMPS would only read the cutoff parameter from the following line:

Zn S S 0.389 109.47 3.00

Can anyone help me to understand this? Any help would be greatly appreciated.


Not sure - I’ve CCd the author, Todd Zeitler, who can hopefully answer.



I think you’ve found a limitation in the implementation of the potential. I originally set it up without considering A-B-A type interactions, but only A-B-C. It would take a little bit of reworking how the input file is read in order to get it to work the way you would like.

Sorry for the inconvenience,