# Pressure scaling more than 1%. This may mean your system is not yet equilibrated **URL:** \ **Category:** User discussions\ **Tags:** mdp-parameters, mdrun\ **Created:** [September 11, 2025, 8:18am UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632 "2025-09-11T08:18:15Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![YGoing](https://avatars.discourse-cdn.com/v4/letter/y/e9bcb4/32.png) [@YGoing](https://gromacs.bioexcel.eu/u/YGoing)\ **Post date:** [September 11, 2025, 8:18am UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/1 "2025-09-11T08:18:15Z")
GROMACS version: 2025.2 I am running NPT simulations for the TIP4P/2005 water model, where Tcoupl=Nose-Hoover (tau\_t=1.0ps), Pcoupl=Parrinello-Rahman (tau\_p=2.0ps), and a time step of dt=1fs. However, when using the new GROMACS version (2025.2), the system quickly becomes unstable and crashes, accompanied by the warning: “Step 147001 Pressure scaling more than 1%. This may mean your system is not yet equilibrated. Use of Parrinello-Rahman pressure coupling during equilibration can lead to simulation instability, and is discouraged”. In contrast, the simulation runs normally with the older version (2022.1). The initial configurations I have chosen have already been equilibriated. I have tested multiple versions and found that this issue occurs starting from GROMACS 2023 onward, but I don’t konw what the reason is. Below are the simulation results obtained using versions 2025.2 and 2022.1: ![Snipaste_2025-09-11_16-17-51](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/2X/5/5bbaecca65aadbfa40ab8084bb35843c6acedaa2.png) ---
**Author:** ![Marius](https://avatars.discourse-cdn.com/v4/letter/m/ed655f/32.png) [@Marius](https://gromacs.bioexcel.eu/u/Marius)\ **Post date:** [September 12, 2025, 9:17am UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/2 "2025-09-12T09:17:18Z")
Hi @YGoing , a possible issue that comes to my mind is the change of the default values for **nstpcouple**. In GMX 2022.1 the default value is 10, in GMX 2025.2 the default is 100. Take a look at this [thread](https://gromacs.bioexcel.eu/t/changes-of-default-nstpcouple-and-nsttcouple-from-version-2022-to-2023/6220/4) for more information. Best, Marius ---
**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:** [September 12, 2025, 9:50am UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/3 "2025-09-12T09:50:46Z")
Indeed. I just found out that you need about 500 integration steps per period for the Parrinello-Rahman barostat. So that would mean nstpcouple=2 for with tau\_p=2 ps. Note that a period of 2 ps is very short. I am currently investigating this issue. ---
**Author:** ![YGoing](https://avatars.discourse-cdn.com/v4/letter/y/e9bcb4/32.png) [@YGoing](https://gromacs.bioexcel.eu/u/YGoing)\ **Post date:** [September 12, 2025, 2:35pm UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/4 "2025-09-12T14:35:16Z")
Thank you! I set nstpcouple=10 and nsttcouple=10, the results are normal now! ---
**Author:** ![YGoing](https://avatars.discourse-cdn.com/v4/letter/y/e9bcb4/32.png) [@YGoing](https://gromacs.bioexcel.eu/u/YGoing)\ **Post date:** [September 12, 2025, 2:38pm UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/5 "2025-09-12T14:38:52Z")
Thank you! By setting the nstcouple value to be consistent with the old version, I’m now getting normal results ---
**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:** [September 15, 2025, 6:06pm UTC](https://gromacs.bioexcel.eu/t/pressure-scaling-more-than-1-this-may-mean-your-system-is-not-yet-equilibrated/12632/6 "2025-09-15T18:06:46Z")
I opened an issue, with high priority: [Too large nstpcouple value for Parrinello-Rahman barostat increases volume fluctuations (#5424) · Issues · GROMACS / GROMACS · GitLab](https://gitlab.com/gromacs/gromacs/-/issues/5424) I did tests for the worst case: a small water box. I uploading a fix for the release 2025 branch. This gives nstpcouple=10 with tau\_p=5.