The force field of NO3-（NO3- consists of N atom in the middle and three O atoms distributed around N atom. There is a bond between each O and N，but there is no bond between O and O.） is“U(r) =K(rN-O - r0_N-O)^2 + H(θ-θ0)^2 + F(rO-O - r0_O-O)^2”in an article. the first term is the term of N-O bond stretching; the second term is the term of O-N-O angle bending and the third term is the none-bonded term between two O in the angle. So what is the pair style about the third term“U=F(rO-O-r0_O-O)^2”in LAMMPS.

The force field of NO3-（NO3- consists of N atom in the middle and three O atoms distributed around N atom. There is a bond between each O and N，but there is no bond between O and O.） is“U® =K(rN-O - r0_N-O)^2 + H(θ-θ0)^2 + F(rO-O - r0_O-O)^2”in an article. the first term is the term of N-O bond stretching; the second term is the term of O-N-O angle bending and the third term is the none-bonded term between two O in the angle. So what is the pair style about the third term“U=F(rO-O-r0_O-O)^2”in LAMMPS.

using a pair style would be wrong, since this only applies to 1-3 pairs, not all O-O pairs. this looks like bonded parameters from a CHARMM-style force field with harmonic bonds and a harmonic angle potential with a Urey-Bradley term.