# creating new residues

**URL:** <https://gromacs.bioexcel.eu/t/creating-new-residues/3901>\
**Category:** User discussions\
**Created:** [March 26, 2022, 1:20pm UTC](https://gromacs.bioexcel.eu/t/creating-new-residues/3901 "2022-03-26T13:20:51Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![deebee](https://avatars.discourse-cdn.com/v4/letter/d/e56c9b/32.png) [@deebee](https://gromacs.bioexcel.eu/u/deebee)\
**Post date:** [March 26, 2022, 1:20pm UTC](https://gromacs.bioexcel.eu/t/creating-new-residues/3901/1 "2022-03-26T13:20:51Z")

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Brief description of tools/files: PTM with GROMOS

Link: [http://vienna-ptm.univie.ac.at/](http://vienna-ptm.univie.ac.at/)

I am trying to run a simulation with methylated residues. I have tried applying a GROMOS forcefield as according to the Vienna PTM procedure but ran into :  
WARNING 1 [file topol.top, line 14360]:  
The GROMOS force fields have been parametrized with a physically  
incorrect multiple-time-stepping scheme for a twin-range cut-off. When  
used with a single-range cut-off (or a correct Trotter  
multiple-time-stepping scheme), physical properties, such as the density,  
might differ from the intended values. Since there are researchers  
actively working on validating GROMOS with modern integrators we have not  
yet removed the GROMOS force fields, but you should be aware of these  
issues and check if molecules in your system are affected before  
proceeding. Further information is available at  
[GROMACS can not reproduce properties with the GROMOS force fields - Redmine #2884 (#2884) · Issues · GROMACS / GROMACS · GitLab](https://gitlab.com/gromacs/gromacs/-/issues/2884), and a longer  
explanation of our decision to remove physically incorrect algorithms can  
be found at [On The Importance of Accurate Algorithms for Reliable Molecular Dynamics Simulations | Theoretical and Computational Chemistry | ChemRxiv | Cambridge Open Engage](https://doi.org/10.26434/chemrxiv.11474583.v1) .

since i have been trying to find a way to manually edit other forcefields as to accomidate my structure ( Adding a Residue to a Force Field). Are there any additional resources or examples that can help me as i develop a system with checks to validate my addition?