# Modify GROMACS after installation **URL:** \ **Category:** User discussions\ **Created:** [October 25, 2023, 2:41pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470 "2023-10-25T14:41:21Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![Richard](https://avatars.discourse-cdn.com/v4/letter/r/f0a364/32.png) [@Richard](https://gromacs.bioexcel.eu/u/Richard)\ **Post date:** [October 25, 2023, 2:41pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/1 "2023-10-25T14:41:21Z")
GROMACS version: 2023.3 GROMACS modification: No I just installed GROMACS on my laptop using the “quick and dirty” installation pathway. Unfortunately, I just realized that I explicitly had to indicate that I want to use my GPU for simulations during the installation process. Is there any possibility to modify GROMACS afterwards? Thanks in advance! ---
**Author:** ![al42and](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/al42and/32/1393_2.png) [@al42and](https://gromacs.bioexcel.eu/u/al42and)\ **Post date:** [October 25, 2023, 4:51pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/2 "2023-10-25T16:51:35Z")
Hi! There is not way to “modify” GROMACS installation, but building GROMACS anew (with GPU enabled) and installing it would work. There is usually no need to uninstall the old version before. ---
**Author:** ![Richard](https://avatars.discourse-cdn.com/v4/letter/r/f0a364/32.png) [@Richard](https://gromacs.bioexcel.eu/u/Richard)\ **Post date:** [October 26, 2023, 2:54pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/3 "2023-10-26T14:54:54Z")
Thank you very much! ---
**Author:** ![Richard](https://avatars.discourse-cdn.com/v4/letter/r/f0a364/32.png) [@Richard](https://gromacs.bioexcel.eu/u/Richard)\ **Post date:** [November 2, 2023, 3:40pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/4 "2023-11-02T15:40:14Z")
There still might be a misunderstanding form my side: In the installation documentation there are two options that I can’t really tell the difference between, because I am not really into the topic (yet): -DGMX\_MPI=on and -DGMX\_GPU=CUDA I want to use the GPU on my laptop (NVIDIA GeForce RTX 3060 Laptop GPU), so do I have to use both options? Under the “GPU-aware MPI support” heading in the docs, there is talk about “simulations using multiple GPUs”, but I just want to use one GPU, namely the one mentioned before. So do I need the cuda toolkit and openmpi? Thanks! ---
**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\ **Post date:** [November 2, 2023, 7:39pm UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/5 "2023-11-02T19:39:01Z")
If you intend to use one GPU, you do not need an external MPI library. ---
**Author:** ![al42and](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/al42and/32/1393_2.png) [@al42and](https://gromacs.bioexcel.eu/u/al42and)\ **Post date:** [November 3, 2023, 12:04am UTC](https://gromacs.bioexcel.eu/t/modify-gromacs-after-installation/7470/6 "2023-11-03T00:04:47Z")
> [@Richard](#): > > There still might be a misunderstanding form my side: In the installation documentation there are two options that I can’t really tell the difference between, because I am not really into the topic (yet): > > -DGMX\_MPI=on > > and > > -DGMX\_GPU=CUDA These two options are completely orthogonal. - CUDA is a library for working with NVIDIA GPUs. You need CUDA to use your GPU. - MPI is a library for parallelizing over multiple computers (can be used on a single machine too). As @jalemkul said, you don’t need `GMX_MPI=ON` for a typical laptop build, and you don’t need to have OpenMPI installed.