# How to determine compressibility?

GROMACS version:2020.2
GROMACS modification: Yes/No

Dear all,

I am trying to run an npt calculation for a pure organic solvent system. How to determine the compressibility then?

Hi,

you can calculate isothermal compressibility

1. by calculating volume fluctuation. < volume fluctuation > _NPT= V * k_B * T * isothermal compressibility (see p 53 chapter Fluctuations Allen Tildesley Book "computer simulation of liquids)
2. by running NVT simulation at slightly different volume and applying
isothermal compressibility = - 1/V * (dV/dP)_T

Kind regards
Alessandra

1 Like

Hello @alevilla ,

Thank you for the suggestions regarding the isothermal compressibility.
In the comment 2, by â€śdifferent volumeâ€ť, how much the volume need to be increased, e.g. dV = ~(0.1)^3 nm^3 will be okay? What is (dV/dP)_T? Is dV = V2-V1? What will be the pressure as is it fluctuating through the NVT similation?

I wonder, if there is any data base for liquids and their isothermal compressibility?

Regards
Ranajit

Hi,

The difference in volume should be tiny ( 1 % ?), but it may also depend on the dimension of the system. Good to have a look in literature there are a lot of systematic study on liquid properties (using MD). In my previous post, dV is the difference between the volume of the two simulations.

If useful, gmx energy can provide isothermal compressibility from volume fluctuation

Concerning a database, you can have a look to CRC Handbook of Chemistry and Physics http://www.hbcponline.com

I hope it helps.
Alessandra

Thank you @alevilla for your comment.