# Can tune\_pme change the Coulomb cutoff to be larger than the simulation box?
**URL:** https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112
**Category:** User discussions
**Created:** [April 5, 2023, 12:11am UTC](https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112 "2023-04-05T00:11:25Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@mjgp](https://gromacs.bioexcel.eu/u/mjgp)
#### Post date: [April 5, 2023, 12:11am UTC](https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112/1 "2023-04-05T00:11:26Z")
GROMACS version: 2022.2
GROMACS modification: No
The optimized Coulomb cutoff found by tune\_pme for my simulaiton is 1.6 nm, but my simulation box has only a 1.5 nm buffer (probably slightly less after equilibration). This sounds like a problem to me. Should I disable tune\_pme, make my simulation box larger, or box, or do something else?
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [April 6, 2023, 3:47pm UTC](https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112/2 "2023-04-06T15:47:01Z")
PME always computes the full, periodic electrostatic interactions, independent of which cut-off is used. So there is no problem with using tune\_pme. It could be that a buffer of 1.5 nm is not sufficient, but that is independent of the parameters for PME.
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### Author:  [@mjgp](https://gromacs.bioexcel.eu/u/mjgp)
#### Post date: [April 6, 2023, 4:21pm UTC](https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112/3 "2023-04-06T16:21:31Z")
If the Coulomb cutoff determined by tune\_pme is larger than the solvent buffer inside the simulation box, should I increase the solvent buffer to match the Coulomb cutoff?
Said another way: How can I tell if the solvent buffer size needs to be increased?
Note that my system is highly charged (140 K+ counterions for neutrality).
Best,
Matthew
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### Author:  [@hess](https://gromacs.bioexcel.eu/u/hess)
#### Post date: [April 6, 2023, 4:25pm UTC](https://gromacs.bioexcel.eu/t/can-tune-pme-change-the-coulomb-cutoff-to-be-larger-than-the-simulation-box/6112/4 "2023-04-06T16:25:34Z")
Electrostatic interactions decay as 1/r. With PME the electrostatic cut-off is irrelevant, except for performance considerations.
What buffer you need if very difficult to determine. That comes own a lot to experience. If you have a high counterion density, there will be a lot of screening and the buffer might not be so critical. Another question is what ion concentration is reasonable.