# Biphasic system

**URL:** <https://gromacs.bioexcel.eu/t/biphasic-system/837>\
**Category:** User discussions\
**Created:** [September 20, 2020, 5:17am UTC](https://gromacs.bioexcel.eu/t/biphasic-system/837 "2020-09-20T05:17:44Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![sujigeorge](https://avatars.discourse-cdn.com/v4/letter/s/858c86/32.png) [@sujigeorge](https://gromacs.bioexcel.eu/u/sujigeorge)\
**Post date:** [September 20, 2020, 5:17am UTC](https://gromacs.bioexcel.eu/t/biphasic-system/837/1 "2020-09-20T05:17:44Z")

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Hi,  
I am building a biphasic system consisting of ethylbenzene and water. Using the gmx insert-molecule i created a box of dimension 15 15 4, with 2754 ethylbenzene molecule the maximum that could fit the box. Then did energy minimization and NVT equilibration. During the NVT equilibration i am facing the issue, the entire structure gets deformed as shown in the attached figure.  
 ![EB-gro](https://europe1.discourse-cdn.com/flex017/uploads/bioexcel1/original/1X/e6e4653c30f85778f16836ba376ebf12e8dbc94d.jpeg)

Any help would be appreciated  
Thanking you

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**Author:** ![Dr\_DBW](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/dr_dbw/32/42_2.png) [@Dr\_DBW](https://gromacs.bioexcel.eu/u/Dr_DBW)\
**Post date:** [September 21, 2020, 12:51am UTC](https://gromacs.bioexcel.eu/t/biphasic-system/837/2 "2020-09-21T00:51:23Z")

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Constant volume simulation means the box size can’t change. That is not a deformation, it is the liquid getting to it’s correct simulation density. Obviously the density of the liquid is higher than what you generated so some areas form a “vacuum”. Best solution is to run NPT for awhile, get the density stable then return to your NVT settings.