# Two different molecules in a box of water **URL:** \ **Category:** User discussions\ **Tags:** simulation-setup\ **Created:** [October 1, 2023, 11:08am UTC](https://gromacs.bioexcel.eu/t/two-different-molecules-in-a-box-of-water/7303 "2023-10-01T11:08:57Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Saeed](https://avatars.discourse-cdn.com/v4/letter/s/e8c25b/32.png) [@Saeed](https://gromacs.bioexcel.eu/u/Saeed)\ **Post date:** [October 1, 2023, 11:08am UTC](https://gromacs.bioexcel.eu/t/two-different-molecules-in-a-box-of-water/7303/1 "2023-10-01T11:08:57Z")
GROMACS version:2023 GROMACS modification: Yes/No Here post your question Hi. I am new to using the Gromacs and would be grateful if you could help me. I want to have 2 molecules (molecules A and B) in a box of water. I tried to use the ligpargen server for each molecule separately but it gave me something like the below: molecule A: opls\_802 C802 12.0110 0.000 A 3.50000E-01 2.76144E-01 molecule B: opls\_802 O802 15.9990 0.000 A 2.96000E-01 8.78640E-01 One atom type (opls\_802) for two different atoms! Probably there is one other way to have two different molecules in one system that I do not know! I would be grateful if you could help me. ---
**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:** [October 3, 2023, 4:38pm UTC](https://gromacs.bioexcel.eu/t/two-different-molecules-in-a-box-of-water/7303/2 "2023-10-03T16:38:36Z")
You need to rename the atomtypes such that their names are unique. ---
**Author:** ![Saeed](https://avatars.discourse-cdn.com/v4/letter/s/e8c25b/32.png) [@Saeed](https://gromacs.bioexcel.eu/u/Saeed)\ **Post date:** [October 4, 2023, 1:08am UTC](https://gromacs.bioexcel.eu/t/two-different-molecules-in-a-box-of-water/7303/3 "2023-10-04T01:08:07Z")
Thanks for your reply! I manually changed the atomtypes and it worked!