# Parrinello-Rahman instability in Gromacs 2023

**URL:** <https://gromacs.bioexcel.eu/t/parrinello-rahman-instability-in-gromacs-2023/5945>\
**Category:** User discussions\
**Tags:** mdrun\
**Created:** [March 9, 2023, 8:11am UTC](https://gromacs.bioexcel.eu/t/parrinello-rahman-instability-in-gromacs-2023/5945 "2023-03-09T08:11:14Z")\
**Posts on this page:** 1\
**Showing post:** 7

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**Author:** ![tenkovin](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/tenkovin/32/2024_2.png) [@tenkovin](https://gromacs.bioexcel.eu/u/tenkovin)\
**Post date:** [March 11, 2023, 8:55am UTC](https://gromacs.bioexcel.eu/t/parrinello-rahman-instability-in-gromacs-2023/5945/7 "2023-03-11T08:55:28Z")

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Yes, i’ve tried 3.0e-5 (1.5 times lower) compressibility for both membrane and water and got only one “Pressure scaling” warning in the very beginning (60000 step) of the simulation. If i keep one of the groups (memb or solv) compressibilities at 4.5e-5, system is less, but still instable. Lowering compressibilities to 1.5e-5 leads to no warnings at all, but i doubt about how physical these values are. Of course, simulations were carried out with nst(p/t)couple equal to 100.  
Also, i’ve tried to calculate compressibility as given in [How to determine compressibility?](https://gromacs.bioexcel.eu/t/how-to-determine-compressibility/787) thread, but came up with numbers twice bigger than 4.5e-5 bar-1. System exploding almost instantaneously with that values being set.  
Thank you for your suggestions and overall help.

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