As the error says it's a Syntax error.

Have you seen this page?

http://statthermo.blogspot.in/2014/06/use-acpype-to-generate-gaff-topologies.html

The answer to this question can be given only researcher who conducted empirical research. I think that all empirical research must have errors. It is not possible to empirically comprehend the essence of reality.

Is your topology empty?

You have to check the order you placed your atoms in the topology

Your ITP file (AIX_GMX.itp) is probably been included twice in your topology.

Check your topol.top file and your ITP files for the duplicate "#include AIX_GMX.itp" line and remove it. It should only be mentioned once in the topol.top file.

I also got this error as I previously put the ligand.itp directly following the protein posre.itp in the topol.top. This worked for the tutorial of protein-ligand complex based on CHARMM ff (http://www.mdtutorials.com/gmx/complex/index.html)

However, when I tried to use AMBER force field, the order of directives in topol.top needed to be adjusted. The moving of the ligand.itp (ligand_GMX.itp generated by acpype) to be directly under the #include "./amber14sb_parmbsc1.ff/forcefield.itp" did work for me.

Then, I got another error "No default Improper Dih. types". For my case, it came from the missing a line in ffbonded.itp which is a file under the

amber14sb_parmbsc1.ff downloaded from

http://www.gromacs.org/Downloads/User_contributions/Force_fields.

This fixing was reported by http://zhenglz.blogspot.com/2017/05/fixing-bugs-in-ff14sb-port-for-gromacs.html But for the ffbonded.itp I have, only the

NA CV CC CT 4 180.00 4.60240 2

was missing.

All above were based on GROMACS 2020.4, amber14sb_parmbsc1.ff, and Antechamber. Antechamber is bundled with AmberTools20. For acpype, I just clone their git https://github.com/alanwilter/acpype

and use only acpype.py with the following command:

acpype.py -i ligand.pdb

I also got this error in Gromacs v2020.4 using CHARMM36-Feb2021 force field. i have prepared ligand topology with different tools such as LigparGen, ATB server, CGenFF server also Acpype webserver but still got same error. i tried many ways but problem is still their.

I checked the topol.top where the forcefield parameters lines in top while in older version the force field parameters defines in last section of topol.top

I found the solution by...

so you have to define your ligand .itp in below forcefield parameters lines.

ex.

; This is a standalone topology file

;

; Created by:

; :-) GROMACS - gmx pdb2gmx, 2020.4 (-:

;

; Executable: /apps/GROMACS/2020.4/bin/gmx_mpi

; Data prefix: /apps/GROMACS/2020.4

; Working dir:

; Command line:

; gmx_mpi pdb2gmx -f protein.pdb -o complex.gro -ignh

; Force field was read from the standard GROMACS share directory.

;

; Include forcefield parameters

#include "charmm36-feb2021.ff/forcefield.itp"

#include "AW_GMX.itp"

.....

[ molecules ]

; Compound #mols

Protein_chain_A 1

LIG 1

SOL 13404