# Adding an organic molecule to OPLS-AA force field **URL:** https://gromacs.bioexcel.eu/t/adding-an-organic-molecule-to-opls-aa-force-field/11006 **Category:** User discussions **Tags:** forcefield **Created:** [January 11, 2025, 10:58am UTC](https://gromacs.bioexcel.eu/t/adding-an-organic-molecule-to-opls-aa-force-field/11006 "2025-01-11T10:58:01Z") **Posts on this page:** 1 **Page:** 1
### Author: ![Nupur\_Nagar](https://avatars.discourse-cdn.com/v4/letter/n/858c86/32.png) [@Nupur\_Nagar](https://gromacs.bioexcel.eu/u/Nupur_Nagar) #### Post date: [January 11, 2025, 10:58am UTC](https://gromacs.bioexcel.eu/t/adding-an-organic-molecule-to-opls-aa-force-field/11006/1 "2025-01-11T10:58:01Z")
GROMACS version: 2023.4/2024.4 GROMACS modification: Yes/No I want to simulate a polymeric organic molecule using OPLS FF. I tried adding an .itp file to the force field but it shows the following error: \*_Back Off! I just backed up topol.top to ./#topol.top.2#_ _Processing chain 1 (2 atoms, 1 residues)_ _Problem with chain definition, or missing terminal residues. This chain does not appear to contain a recognized chain molecule. If this is incorrect, you can edit residuetypes.dat to modify the behavior._ _8 out of 8 lines of specbond.dat converted successfully_ _-------------------------------------------------------_ _Program: gmx pdb2gmx, version 2021.4_ _Source file: src/gromacs/gmxpreprocess/resall.cpp (line 597)_ _Fatal error:_ _Residue ‘MEH’ not found in residue topology database_ _For more information and tips for troubleshooting, please check the GROMACS_ _website at [Common Errors — GROMACS webpage https://www.gromacs.org documentation](http://www.gromacs.org/Documentation/Errors)_ _-------------------------------------------------------_ _--------------------------------------------------------------------------_ _MPI\_ABORT was invoked on rank 0 in communicator MPI\_COMM\_WORLD_ _with errorcode 1._ - All the parameters were generated using LigParGen and rechecked with Gaussian. However do I need to make an .rtp file for the same? Please let me know if there is another way of introducing a new organic molecule to a forcefield.