Amino acids are not recognized by GROMACS

GROMACS version: 5.1.2
GROMACS modification: No
Here post your question

I want to do protein-peptide simulation. I am following the steps same as whole protein simulation. However, during topology setup, the amino acids did not get recognized by the system. I am using gromos54a7 as force field.

The peptide is composed of: BEZ NLE KR OAR
The details:
BEZ- Benzoic acid
NLE- Norleucine (isomer of leucine)
KR- lysine, arginine
OAR - amino acid domain

I get few warnings from the system:
Starting residue BEZ1 in chain not identified as Protein/RNA/DNA
Starting residue NLE2 in chain not identified as Protein/RNA/DNA
Starting residue OAR5 in chain not identified as Protein/RNA/DNA

Fatal error:
Residue ‘BEZ’ not found in residue topology database
Residue ‘NLE’ not found in residue topology database

How I can solve these problems? I want to keep BEZ (even though it is not amino acid residue) in the peptide structure as it plays a role in structural conformational.

Your helps are greatly appreciated. Thank you.

Nonstandard species are generally not covered by most force fields, as the fatal error tells you - pdb2gmx does not know what the residue is. Follow Adding a Residue to a Force Field — GROMACS 2022.1 documentation and if you have to do any complex parametrization, it may take quite some effort.