Improper Dihedrals

For impropers in the lammps data file:

improperid impropertyp atom1 atom2 atom3 atom4

atom 3 is the central atom

What vmd gives (7162) is correct.

What you have is a phenylene ring and planarity is enforced (using improper torsions although the angle parameters if defined correctly defined will do the job) such that the ring is coplanar.

If you want to define your own improper

ie “bonds I-J, I-K, I-L would exist for an improper to be defined between the 4 atoms” is not the case then you have to manually put that in the data file

I hope this clears up the issue !.