# How can I add molecules to an equilibrated system without messing PBCs? **URL:** \ **Category:** User discussions\ **Tags:** simulation-setup\ **Created:** [January 16, 2025, 2:20pm UTC](https://gromacs.bioexcel.eu/t/how-can-i-add-molecules-to-an-equilibrated-system-without-messing-pbcs/11051 "2025-01-16T14:20:28Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Lois](https://avatars.discourse-cdn.com/v4/letter/l/5f8ce5/32.png) [@Lois](https://gromacs.bioexcel.eu/u/Lois)\ **Post date:** [January 16, 2025, 2:20pm UTC](https://gromacs.bioexcel.eu/t/how-can-i-add-molecules-to-an-equilibrated-system-without-messing-pbcs/11051/1 "2025-01-16T14:20:28Z")
GROMACS version: 2018 Hi all, I’m doing atomistic simulations of polyisoprene with plasticizer additives to study phase behaviour. I am first equilibrating the PI which takes a long time and then I want to add the additives to the PI to observe any phase separation. The issue is that the PI is very dense and insert-molecules can’t insert the additives. If I increase the box size then there is a problem with the pbc and the simulation will explode. I have heard some people simulating with a cavity in their box where molecules can be inserted after equilibration, does anyone have any experience with this in GROMACS? Or any alternative solutions to this problem? Thanks ---
**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\ **Post date:** [January 16, 2025, 3:22pm UTC](https://gromacs.bioexcel.eu/t/how-can-i-add-molecules-to-an-equilibrated-system-without-messing-pbcs/11051/2 "2025-01-16T15:22:01Z")
You can use the `-scale` option with `gmx insert-molecules`. If you go very low you will then need to use alchemical FEP methods to start with downscaled interactions, which you turn on. But if a `-scale 0.4` (default 0.57), or perhaps a little lower, is enough, you might manage with normal EM and equilibration.