# Insert the simulation box

**URL:** <https://gromacs.bioexcel.eu/t/insert-the-simulation-box/12452>\
**Category:** User discussions\
**Tags:** simulation-setup\
**Created:** [July 29, 2025, 9:53am UTC](https://gromacs.bioexcel.eu/t/insert-the-simulation-box/12452 "2025-07-29T09:53:46Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![sanket\_singare](https://avatars.discourse-cdn.com/v4/letter/s/96bed5/32.png) [@sanket\_singare](https://gromacs.bioexcel.eu/u/sanket_singare)\
**Post date:** [July 29, 2025, 9:53am UTC](https://gromacs.bioexcel.eu/t/insert-the-simulation-box/12452/1 "2025-07-29T09:53:46Z")

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GROMACS version:  
GROMACS modification: Yes/No  
Here post your question  
hello users  
how much amount of atoms or molecules should i insert in known simulation box to achieve the the known density?  
example : if i known the density of ethylene density 1.113 g/mL at 25 °C experimentally, but i need to do md simulation studies and i need to achieve the density which has been reported by paper. so how much amount of atoms or molecules, as well as how much amount of simulation box need to take? (how should pack in packmol, if anyone known)  
while doing the simulation the constrains as well as grid size can affect the density (md parameters)

please resolve the issue