Pair_style for Harmonic potential?

Hello sir,
i want interaction between active brownian particles and substrate particles in 2 dimension… but I did not found this potential in unbond style
harmonic potential =1/2 k(r-r0)^2
but I found it in bond style…
plz help me to consider this potential…
thank you

As you have found, there is no pair style implemented for that.
You would either write a custom pair style (e.g. based on lj/cut) to realize this ( 3. Modifying & extending LAMMPS — LAMMPS documentation )
implement this via pair style python with a python class ( pair_style python command — LAMMPS documentation )
or use a tabulated potential ( pair_style table command — LAMMPS documentation ).
The tabulation can also created from the Python variant using the pair_write command — LAMMPS documentation

Please carefully review the linked documentation.

Or you could create a feature request issue describing the specifics at: Issues · lammps/lammps · GitHub
and hope that somebody volunteers to implement the potential you are looking for.

Thank you so much for your kind suggestions…