# High pressure in x and y axis in a buble in a nano channel

**URL:** <https://gromacs.bioexcel.eu/t/high-pressure-in-x-and-y-axis-in-a-buble-in-a-nano-channel/13077>\
**Category:** User discussions\
**Tags:** mdp-parameters, mdrun, simulation-setup\
**Created:** [January 3, 2026, 3:16am UTC](https://gromacs.bioexcel.eu/t/high-pressure-in-x-and-y-axis-in-a-buble-in-a-nano-channel/13077 "2026-01-03T03:16:50Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Mahnaz](https://avatars.discourse-cdn.com/v4/letter/m/2acd7d/32.png) [@Mahnaz](https://gromacs.bioexcel.eu/u/Mahnaz)\
**Post date:** [January 3, 2026, 3:16am UTC](https://gromacs.bioexcel.eu/t/high-pressure-in-x-and-y-axis-in-a-buble-in-a-nano-channel/13077/1 "2026-01-03T03:16:50Z")

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GROMACS version: gromacs/2024.4  
GROMACS modification: No  
Hi everyone,

I am simulating a mixed H₂O/H₂ system confined in an Al₂O₃ nanochannel. The channel walls lie in the X–Y plane, and the confinement direction is along Z.

I would like to apply anisotropic pressure coupling such that the pressure inside the nanochannel is **25 bar in the Z direction** and **1 bar in the X and Y directions**.

; Pressure coupling  
pcoupl = Parrinello-Rahman ; Pressure coupling method  
pcoupltype = semiisotropic ; Semiisotropic pressure coupling  
tau\_p = 2 ; Pressure coupling time constant (ps)  
ref\_p = 1 25.0 ; Reference pressures: 1 bar in x/y, 25.0 bar in z  
compressibility = 4.5e-20 4.5e-5 ; Compressibility: near-zero in x/y, standard in z  
nstpcouple = 20 ; Frequency for pressure  
refcoord\_scaling = COM

After equilibration, the pressure along the **Z direction converges to 25 bar** , which is the expected behavior.  
However, the pressures along the **X and Y directions increase to very large values** , as shown below:

Pressure (bar)  
3.02652e+04 -6.38528e+00 -8.65890e-01  
-6.38820e+00 3.01417e+04 1.50278e+01  
-8.57413e-01 1.50357e+01 2.37052e+01

Can someone please help me understand the origin of these high pressures in the X and Y directions?  
Is this behavior expected for a nanochannel system under periodic boundary conditions?

Thank you in advance for your time and help.

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**Author:** ![MichelePellegrino](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/michelepellegrino/32/628_2.png) [@MichelePellegrino](https://gromacs.bioexcel.eu/u/MichelePellegrino)\
**Post date:** [January 3, 2026, 8:21pm UTC](https://gromacs.bioexcel.eu/t/high-pressure-in-x-and-y-axis-in-a-buble-in-a-nano-channel/13077/2 "2026-01-03T20:21:08Z")

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Hi,

> compressibility = 4.5e-20 4.5e-5 ; Compressibility: near-zero in x/y, standard in z

I am not really sure how one could impose _any_ meaningful pressure in with such a small compressibility. It’s basically a NPTA (constant area) ensemble. Have you tried increasing the compressibility in x/y (assuming it’s what you actually want to do)?

P.S. the high values can be also due to the fact that walls are frozen/restrained (are they)?

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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:** [January 5, 2026, 2:16pm UTC](https://gromacs.bioexcel.eu/t/high-pressure-in-x-and-y-axis-in-a-buble-in-a-nano-channel/13077/3 "2026-01-05T14:16:26Z")

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You can not use pressure coupling, which acts on the whole system, to enforce a pressure in part of the system, in particular when there are solid structures involved. Furthermore, I don’t understand what you want to achieve. A gas, or a liquid, can not have different pressures along different dimensions.