what error did You see (give a full quote)? I can't open Your top file...and You can also provide an itp file to understand the reason (it should be cropped).

@Alex Rayevsky

Thanks for your response. I am attaching the top and itp file.

The error is

"ATOM TYPE CA NOT FOUND"

please, try this tip - rename 'lig' to 'SPECS_3_H' in itp or its better to rename vise versa in all files (LIG is much more suitable and short enough). What is the result?

@Alex Rayevsky

I got the same error, I also gave a try with opls ff the same kind of error:

ERROR 1 [file lig.itp, line 24]:

Atomtype opls_x not found

in private )

I recommend you use acpype. This tool is based on amb2gmx and works well with 5.x versions of gromacs.

try modifying the topology file. the topology file shown here does not include the force field ##############################################

; Include forcefield parameters

#include "amber99sb.ff/forcefield.itp"

; Include Ligand topology

#include "ligand.itp"

; Include water topology

#include "amber99sb.ff/tip3p.itp"

#ifdef POSRES_WATER

; Position restraint for each water oxygen

[ position_restraints ]

; i funct fcx fcy fcz

1 1 1000 1000 1000

#endif

; Include topology for ions

#include "amber99sb.ff/ions.itp"

[ system ]

; Name

Ligand in water

[ molecules ]

; Compound #mols

LIG 1

##########################################