# 2D system treatment

**URL:** <https://gromacs.bioexcel.eu/t/2d-system-treatment/5292>\
**Category:** User discussions\
**Tags:** mdrun\
**Created:** [November 22, 2022, 1:27am UTC](https://gromacs.bioexcel.eu/t/2d-system-treatment/5292 "2022-11-22T01:27:45Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![hejamu](https://avatars.discourse-cdn.com/v4/letter/h/bbe5ce/32.png) [@hejamu](https://gromacs.bioexcel.eu/u/hejamu)\
**Post date:** [November 22, 2022, 1:27am UTC](https://gromacs.bioexcel.eu/t/2d-system-treatment/5292/1 "2022-11-22T01:27:45Z")

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Hi developers,

setting up 2D slab systems we noticed something ‘unexpected’.

We prepared 2 systems, both are SPC/E water confined between graphene sheets that are frozen.

System 1 has `nwalls = 2` with dummy atomtypes (charge and LJ params set to 0), `wall-ewald-zfac = 3` and `pbc = xy`

System 2 has no walls and `pbc = xyz` but explicit vacuum that is two times larger than the slab (equaling the behaviour of `wall-ewald-zfac = 3`

Both systems have `ewald-geometry = 3dc` set.

In theory, the two systems should yield the same results as electrostatics and LJ interactions should be treated the same. Yet, the systems differ in nearly all observables.

We are looking into this right now, but maybe we are missing something here?

Best

Henrik Jaeger

EDIT: Tested with 2021.4 and 2022.4

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**Author:** ![hess](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/hess/32/416_2.png) [@hess](https://gromacs.bioexcel.eu/u/hess)\
**Post date:** [November 22, 2022, 7:35am UTC](https://gromacs.bioexcel.eu/t/2d-system-treatment/5292/2 "2022-11-22T07:35:21Z")

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Do the systems have the same initial coordinates? And do they differ already at step 0 or after many steps?

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**Author:** ![MayankS](https://avatars.discourse-cdn.com/v4/letter/m/dfb087/32.png) [@MayankS](https://gromacs.bioexcel.eu/u/MayankS)\
**Post date:** [February 14, 2025, 5:43pm UTC](https://gromacs.bioexcel.eu/t/2d-system-treatment/5292/3 "2025-02-14T17:43:48Z")

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Dear Henrik,

I somehow ended up at this discussion. It would be great if you could post a short update on what you found regarding this issue.

Thanks in advance,  
Mayank