# How to use

**URL:** <https://gromacs.bioexcel.eu/t/how-to-use/8175>\
**Category:** User discussions\
**Created:** [January 30, 2024, 9:09am UTC](https://gromacs.bioexcel.eu/t/how-to-use/8175 "2024-01-30T09:09:07Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![xieliqiang](https://avatars.discourse-cdn.com/v4/letter/x/ea666f/32.png) [@xieliqiang](https://gromacs.bioexcel.eu/u/xieliqiang)\
**Post date:** [January 30, 2024, 9:09am UTC](https://gromacs.bioexcel.eu/t/how-to-use/8175/1 "2024-01-30T09:09:07Z")

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GROMACS version:2023.1  
GROMACS modification: No  
Here post your question  
hi,everyone,  
i have a micelle composed of 40 molecules A, and i have a drug molecule B. i wang to caculate the rdf function between the center of mass (com) of the micelle and drug B, but i can not get a proper result.( the drug was on the surface of the miclle ,but the rdf was not right.

my rdf is caculated by :  
gmx rdf -f trajout.xtc -s md.tpr -ref “com of group A” -sel “com of group B” -pbc -bin 0.1

thank you in advace.