# VdW modifier for CHARMM LJPME simulations?

**URL:** <https://gromacs.bioexcel.eu/t/vdw-modifier-for-charmm-ljpme-simulations/2622>\
**Category:** User discussions\
**Tags:** forcefield, mdp-parameters\
**Created:** [August 2, 2021, 9:23am UTC](https://gromacs.bioexcel.eu/t/vdw-modifier-for-charmm-ljpme-simulations/2622 "2021-08-02T09:23:33Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![siyoungkim](https://avatars.discourse-cdn.com/v4/letter/s/b2d939/32.png) [@siyoungkim](https://gromacs.bioexcel.eu/u/siyoungkim)\
**Post date:** [August 2, 2021, 9:23am UTC](https://gromacs.bioexcel.eu/t/vdw-modifier-for-charmm-ljpme-simulations/2622/1 "2021-08-02T09:23:33Z")

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GROMACS version: GROMACS 2020  
GROMACS modification: No

Hello,

I’ve been using a VdW modifier of force-switching for simulations using the CHARMM36 force field (C36). I’d like to run simulations with C36/LJ-PME, which was released this year. According to the user manual ([Long Range Van der Waals interactions — GROMACS 2021.2 documentation](https://manual.gromacs.org/documentation/current/reference-manual/functions/long-range-vdw.html)), a VdW-modifier of Potential-Shift is recommended. In LJ-PME simulations, I assume I should no longer use a force-switching function because LJ interactions outside of the real-distance cutoff still should be considered.

How does a VdW-modifier affect LJPME simulations?  
Should I use Potential-Shift for C36/LJ-PME simulations?  
Will Potential-Shift and None give you identical results in LJ-PME simulations?  
(Forces will be the same for both two options, therefore, chooisng either will be fine?)

Thanks!  
Best,  
Siyoung