# Make Index of non hydrogen Atom in Gromacs **URL:** \ **Category:** User discussions\ **Created:** [October 30, 2024, 5:46am UTC](https://gromacs.bioexcel.eu/t/make-index-of-non-hydrogen-atom-in-gromacs/10448 "2024-10-30T05:46:37Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![aydatazh](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/aydatazh/32/3346_2.png) [@aydatazh](https://gromacs.bioexcel.eu/u/aydatazh)\ **Post date:** [October 30, 2024, 5:46am UTC](https://gromacs.bioexcel.eu/t/make-index-of-non-hydrogen-atom-in-gromacs/10448/1 "2024-10-30T05:46:38Z")
GROMACS version: 2024.2. GROMACS modification: No Hi, In this command, why should i add \* after H? Is that related to any chemmical concept? gmx make\_ndx -f jz4.gro -o index\_jz4.ndx … > 0 & ! a H\* > q ---
**Author:** ![MagnusL](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/magnusl/32/2380_2.png) [@MagnusL](https://gromacs.bioexcel.eu/u/MagnusL)\ **Post date:** [October 30, 2024, 7:41am UTC](https://gromacs.bioexcel.eu/t/make-index-of-non-hydrogen-atom-in-gromacs/10448/2 "2024-10-30T07:41:48Z")
The syntax of `gmx make_ndx` should be better documented, indeed. The `*` does not have anything to do with chemistry, but it is a selection wildcard, meaning that you select all atoms (by name) that begins with `H` in this case. Then the `!` inverts that selection. So, the final selection will be all atoms in group 0 (System) that does not have atom name beginning with `H`. ---
**Author:** ![aydatazh](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/aydatazh/32/3346_2.png) [@aydatazh](https://gromacs.bioexcel.eu/u/aydatazh)\ **Post date:** [October 30, 2024, 8:28am UTC](https://gromacs.bioexcel.eu/t/make-index-of-non-hydrogen-atom-in-gromacs/10448/3 "2024-10-30T08:28:13Z")
tnx 👍🙏