# Compressibility factor in non-aqueous solvents **URL:** \ **Category:** User discussions\ **Tags:** mdrun, simulation-setup\ **Created:** [December 14, 2023, 5:26pm UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865 "2023-12-14T17:26:54Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Ali\_Jafari](https://avatars.discourse-cdn.com/v4/letter/a/ee59a6/32.png) [@Ali\_Jafari](https://gromacs.bioexcel.eu/u/Ali_Jafari)\ **Post date:** [December 14, 2023, 5:26pm UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/1 "2023-12-14T17:26:54Z")
GROMACS version:2021.7 GROMACS modification: No Here post your question Hello everyone, I’ve been trying to perform MD simulation for a protein-ligand complex in non-aqueous solvent; Since this solvent has a different isothermal compressibility factor to (1.3e-4bar-1) to that of water, should I use this value in my .mdp file or is it OK to use the compressibility factor of water? I appreciate any help in advence. ---
**Author:** ![yehon](https://avatars.discourse-cdn.com/v4/letter/y/e99b99/32.png) [@yehon](https://gromacs.bioexcel.eu/u/yehon)\ **Post date:** [December 15, 2023, 1:54pm UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/2 "2023-12-15T13:54:47Z")
Hi Ali, You can find the answer [here](https://gromacs.bioexcel.eu/t/on-compressibility-under-pressure-coupling-section-of-mdp/5421). When you change the compressibility, do you get different properties? ---
**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:** [December 15, 2023, 5:54pm UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/3 "2023-12-15T17:54:21Z")
Yes, use the value for your solvent. But using an incorrect compressibility with effectively only give a different coupling time than the tau you supply. ---
**Author:** ![Ali\_Jafari](https://avatars.discourse-cdn.com/v4/letter/a/ee59a6/32.png) [@Ali\_Jafari](https://gromacs.bioexcel.eu/u/Ali_Jafari)\ **Post date:** [December 16, 2023, 8:33am UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/4 "2023-12-16T08:33:00Z")
Thank you @hess is there any reference to find the right value for tau? or is there any way to determine it myself? According to some research articles the experimentally measured value for acetonitrile at 45C is nearly 1.3e-4 (bar-1) but unfortunately I have not found anything related to tau and as far as I know almost all MD simulations related to protein-ligand complex have been biased in one way or another towards aqueous medium. ---
**Author:** ![Ali\_Jafari](https://avatars.discourse-cdn.com/v4/letter/a/ee59a6/32.png) [@Ali\_Jafari](https://gromacs.bioexcel.eu/u/Ali_Jafari)\ **Post date:** [December 16, 2023, 8:43am UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/5 "2023-12-16T08:43:10Z")
Thank you @yehon Since I have not found any reference to compare my results with, I honestly don’t know; but after equilibration in NVT and NPT ensembles the density of system is nearly equal to that of acetonitrile ( this actually happens when I use 4.5e-5) but when I change the compressibility to 1.3e-4 I my system does not equilibriate properly resulting in an error in MD production run. I don’t know if the density of the system counts as a reliable way :) ---
**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:** [December 21, 2023, 11:34am UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/6 "2023-12-21T11:34:09Z")
You choose tau\_p yourself. For the c-rescale barostat I recommend using 2 ps, for parinello-rahman at least 5 ps. ---
**Author:** ![Ali\_Jafari](https://avatars.discourse-cdn.com/v4/letter/a/ee59a6/32.png) [@Ali\_Jafari](https://gromacs.bioexcel.eu/u/Ali_Jafari)\ **Post date:** [December 22, 2023, 12:05pm UTC](https://gromacs.bioexcel.eu/t/compressibility-factor-in-non-aqueous-solvents/7865/7 "2023-12-22T12:05:48Z")
I really appreciate your help