# Hydorder output

**URL:** <https://gromacs.bioexcel.eu/t/hydorder-output/1719>\
**Category:** User discussions\
**Created:** [March 9, 2021, 11:01am UTC](https://gromacs.bioexcel.eu/t/hydorder-output/1719 "2021-03-09T11:01:57Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![yasser.almeida](https://avatars.discourse-cdn.com/v4/letter/y/76d3ee/32.png) [@yasser.almeida](https://gromacs.bioexcel.eu/u/yasser.almeida)\
**Post date:** [March 9, 2021, 11:01am UTC](https://gromacs.bioexcel.eu/t/hydorder-output/1719/1 "2021-03-09T11:01:57Z")

</div>

GROMACS version: 2020.1  
GROMACS modification: No

Hi,

I want to use the `hydorder` tool to compute the tetrahedral order parameter of water molecules. When using:

`gmx hydorder -s box.tpr -f traj.xtc -b 5000 -n index_oxygens.ndx -o file1.xpm -or file1.out`

it gives…

```
Taking z axis as normal to the membrane

-------------------------------------------------------
Program: gmx hydorder, version 2020.1-Ubuntu-2020.1-1
Source file: src/gromacs/gmxana/gmx_hydorder.cpp (line 695)

Fatal error:
No or not correct number (2) of output-files: 1

```

But, If I use (according to [How to use gmx hydorder to calculate tetrahedral order parameter of water](https://gromacs.bioexcel.eu/t/how-to-use-gmx-hydorder-to-calculate-tetrahedral-order-parameter-of-water/1685)):

`gmx hydorder -s ../box.tpr -f ../traj.xtc -b 5000 -n index_oxygens.ndx -o file1.xpm file2.xpm -or file1.out file2.out`

… it starts reading the frames (very slowly though).

Why do I need more than 2 output files with options `-o` and `-or`? Also, my systems is just a simple water box, not a membrane system.

Thanks in advance.

Best,  
-Yasser

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<div class="post-metadata">

**Author:** ![Valentina](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/valentina/32/725_2.png) [@Valentina](https://gromacs.bioexcel.eu/u/Valentina)\
**Post date:** [June 3, 2021, 11:48am UTC](https://gromacs.bioexcel.eu/t/hydorder-output/1719/2 "2021-06-03T11:48:29Z")

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I think I am a bit late with a reply, but also came across this error. Basically, as you say, you need to put 2 file names after the -o or -or option

> gmx hydorder -n orderICE.ndx -b 4500 -o test\_1 test\_2 -or test\_4 test\_5  
> Hope this helps someone also searching