Thank you for the clarification.  There’s a good chance there is more to this 
scenario   but I recall making polymers ( with Avogagdo ) , using pdb2gmx to 
create the top ( then itp )  and sometimes having the same error as the OP 
mentioned. The errors were eliminated by adding the unfound type with 
atomname2type.n2t

I should be able to post the sequence of events if you think it useful to 
others.

Paul
> On Dec 15, 2019, at 11:39 AM, Justin Lemkul <[email protected]> wrote:
> 
> 
> 
> On 12/14/19 1:14 PM, Paul Buscemi wrote:
>> If the itp is correct then modify atomnames2types to add the correct bonds 
>> and bond lengths
> 
> The atomname2type.n2t file is only relevant with x2top, but the OP already 
> has a topology, so this is not the issue. If you mean the .atp file, then 
> this too is not relevant because pdb2gmx is the only program that reads this.
> 
> -Justin
> 
>> PB
>> 
>>> On Dec 13, 2019, at 7:52 PM, Justin Lemkul <[email protected]> wrote:
>>> 
>>> 
>>> 
>>>> On 12/13/19 2:18 AM, Muthusankar wrote:
>>>> Dear Gromacs users,
>>>> I am simulating a protein-ligand complex and performing the grompp command
>>>> before adding ions to the system. I got the error.
>>>> *Fatal error: *(file: ligand.itp)
>>>> Atomtype *CH2 not found*.
>>>> *command used:*
>>>> gmx grompp -f ions.mdp -c protein_box.gro -p protein.top -o ions.tpr
>>>> 
>>>> Please guide me, How to rectify the problem.
>>>> 
>>> This means you're trying to use an atom type that your force field doesn't 
>>> recognize. Either you're mixing and matching force fields (never do this) 
>>> or your ligand topology relies on new atom types that should be introduced 
>>> into the force field, in which case the source of the ligand topology 
>>> (server, etc.) should provide that information.
>>> 
>>> -Justin
>>> 
>>> -- 
>>> ==================================================
>>> 
>>> Justin A. Lemkul, Ph.D.
>>> Assistant Professor
>>> Office: 301 Fralin Hall
>>> Lab: 303 Engel Hall
>>> 
>>> Virginia Tech Department of Biochemistry
>>> 340 West Campus Dr.
>>> Blacksburg, VA 24061
>>> 
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>>> http://www.thelemkullab.com
>>> 
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> 
> -- 
> ==================================================
> 
> Justin A. Lemkul, Ph.D.
> Assistant Professor
> Office: 301 Fralin Hall
> Lab: 303 Engel Hall
> 
> Virginia Tech Department of Biochemistry
> 340 West Campus Dr.
> Blacksburg, VA 24061
> 
> [email protected] | (540) 231-3129
> http://www.thelemkullab.com
> 
> ==================================================
> 
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