Add Ashbaugh Hatch potential to gmx source code

GROMACS version: 2019.6
GROMACS modification: Yes
I am not able to find the cpp/h file where the main LJ calculation has been performed.
I would like to add the Ashbaugh-Hatch potential to gmx by modifying the source code.

V_lj + (1-L) * epsilon if x<2^(1/6)sigma
else, L * V_lj.

One can also tweak the table potential input a bit also to include the 3rd parameter (L).
Or one can add a new force type similar to LJ.
Please point me to the file(s) needed to edit so that I can at least try modifying the source code.