# How to generate a GROMACS topology (.itp) for a silver nanocluster using the INTERFACE Force Field?

**URL:** <https://gromacs.bioexcel.eu/t/how-to-generate-a-gromacs-topology-itp-for-a-silver-nanocluster-using-the-interface-force-field/13740>\
**Category:** User discussions\
**Tags:** forcefield\
**Created:** [September 30, 2026, 5:48pm UTC](https://gromacs.bioexcel.eu/t/how-to-generate-a-gromacs-topology-itp-for-a-silver-nanocluster-using-the-interface-force-field/13740 "2026-09-30T17:48:45Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![supravadas728](https://avatars.discourse-cdn.com/v4/letter/s/85f322/32.png) [@supravadas728](https://gromacs.bioexcel.eu/u/supravadas728)\
**Post date:** [September 30, 2026, 5:48pm UTC](https://gromacs.bioexcel.eu/t/how-to-generate-a-gromacs-topology-itp-for-a-silver-nanocluster-using-the-interface-force-field/13740/1 "2026-09-30T17:48:45Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Hello everyone,

I am currently having difficulty understanding the correct topology/parameterization workflow for the metal nanocluster.

My system consists of a protein interacting with a metal nanocluster of approximately 6 nm diameter. I would like to use the INTERFACE Force Field (IFF) for the metal nanocluster.

Following a suggestion, I used the CHARMM-GUI Nanomaterial Modeler and selected GROMACS as the target MD engine. The generated GROMACS directory contains files such as:

- `step3_input.gro`
- `step3_input.pdb`
- `step3_input.psf`
- `topol.top`
- `toppar/NMA.itp`
- `toppar/forcefield.itp`
- `toppar/TIP3.itp`
- `toppar/SOD.itp`
- `toppar/CLA.itp`

My main confusion is whether `NMA.itp` is already the topology for the generated nanocluster, with the required IFF parameters defined in `forcefield.itp`, or whether I need to combine information from these files to create a separate final `metal.itp` file.

I have also explored ACPYPE, but I understand that generating parameters through ACPYPE may not necessarily reproduce the INTERFACE Force Field parameterization that I want to use.

Could someone please clarify:

1. Which of the CHARMM-GUI-generated files contains the actual topology of the Ag nanocluster?
2. Are the atom types and non-bonded parameters already defined in the generated force-field files?
3. Should `NMA.itp` be included directly in `topol.top` rather than creating another `.itp` file?
4. What is the recommended procedure for integrating this IFF-based metal nanocluster topology with a CHARMM36 protein topology in GROMACS?
5. Are there any additional steps required when using a larger nanocluster with IFF?

I would appreciate any guidance or examples from users who have worked with metal nanoparticles and the INTERFACE Force Field in GROMACS.

I can provide the relevant sections of `NMA.itp`, `forcefield.itp`, and `topol.top` if needed.

Thank you.