# Calculation of diffusion coefficient

**URL:** <https://gromacs.bioexcel.eu/t/calculation-of-diffusion-coefficient/5414>\
**Category:** User discussions\
**Tags:** analysis-tools\
**Created:** [December 10, 2022, 3:21pm UTC](https://gromacs.bioexcel.eu/t/calculation-of-diffusion-coefficient/5414 "2022-12-10T15:21:13Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Narges](https://avatars.discourse-cdn.com/v4/letter/n/7993a0/32.png) [@Narges](https://gromacs.bioexcel.eu/u/Narges)\
**Post date:** [December 10, 2022, 3:21pm UTC](https://gromacs.bioexcel.eu/t/calculation-of-diffusion-coefficient/5414/1 "2022-12-10T15:21:13Z")

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GROMACS version:2022  
GROMACS modification: Yes/No  
Hi all,  
I am trying to calculate diffusion coefficient of a gas molecule in a glassy polymer using ‘gmx msd’.  
I have a problem while running the command: “gmx msd -f NPT.trr -s NPT.tpr -o MSD.xvg”.  
After ‘Specify any number of selections for option ‘sel’ ‘ I face with ‘Selection ‘3’ parsed’ and I have to press Ctrl+D to end. Please suggest me how to fix the problem.