# Free Energy Tutorial-Error during NPT equilibration
**URL:** https://gromacs.bioexcel.eu/t/free-energy-tutorial-error-during-npt-equilibration/2224
**Category:** User discussions
**Created:** [May 26, 2021, 1:37am UTC](https://gromacs.bioexcel.eu/t/free-energy-tutorial-error-during-npt-equilibration/2224 "2021-05-26T01:37:35Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@mkubasik](https://gromacs.bioexcel.eu/u/mkubasik)
#### Post date: [May 26, 2021, 1:37am UTC](https://gromacs.bioexcel.eu/t/free-energy-tutorial-error-during-npt-equilibration/2224/1 "2021-05-26T01:37:35Z")
GROMACS version: 5.07/2021.1/2021.2
GROMACS modification: Yes/No
Dear Gromacs Users,
I’m having difficulty with the pressure equilibration step in Prof. Lemkul’s Free Energy tutorial found at [Free Energy Calculations](http://www.mdtutorials.com/gmx/free_energy/index.html).
During NPT equilibration, I get a fatal error:
The box size in direction X (2.976942) times the triclinic skew factor
(0.816497) is too small for a cut-off of 1.216000 with 2 domain decomposition
cells,
I do NOT get this error when I use gmx 5.07, but I do get the error with 2021.1 and 2021.2.
I have tried using the Berendsen barostat and lengthening tau\_p, but neither helped.
Both 5.07 and 2021.1 seem to distort the cubic box, starting with the EM step.
Any suggestions?
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### Author:  [@mkubasik](https://gromacs.bioexcel.eu/u/mkubasik)
#### Post date: [May 26, 2021, 6:15pm UTC](https://gromacs.bioexcel.eu/t/free-energy-tutorial-error-during-npt-equilibration/2224/2 "2021-05-26T18:15:21Z")
Sorry, everyone, my mistake. Now that I have changed the domain decomposition parameters, the tutorial is running.