# Need a run input file to select atom types

**URL:** <https://gromacs.bioexcel.eu/t/need-a-run-input-file-to-select-atom-types/4171>\
**Category:** User discussions\
**Created:** [May 15, 2022, 4:05pm UTC](https://gromacs.bioexcel.eu/t/need-a-run-input-file-to-select-atom-types/4171 "2022-05-15T16:05:42Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Chintu](https://avatars.discourse-cdn.com/v4/letter/c/cdc98d/32.png) [@Chintu](https://gromacs.bioexcel.eu/u/Chintu)\
**Post date:** [May 15, 2022, 4:05pm UTC](https://gromacs.bioexcel.eu/t/need-a-run-input-file-to-select-atom-types/4171/1 "2022-05-15T16:05:42Z")

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I want to make index file according to atom type. I run gmx make\_ndx -f .gro -o .ndx . Then I went with the option ‘t’. But I am getting an error “need a run input file to select atom types”. Can anyone help me with this? what does it mean?

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**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:** [May 15, 2022, 6:39pm UTC](https://gromacs.bioexcel.eu/t/need-a-run-input-file-to-select-atom-types/4171/2 "2022-05-15T18:39:36Z")

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It means you need a `.tpr` file (which is also called a “run input file”) because it is the only file that associates topological information (atom types) with global atom numbers. Coordinate files only contain residue and atom names, thus you can only make selections based on these properties with `make_ndx`.