# Creating a cross\_section.xml file

**URL:** <https://openmc.discourse.group/t/creating-a-cross-section-xml-file/3451>\
**Category:** User Support\
**Created:** [September 21, 2023, 4:17pm UTC](https://openmc.discourse.group/t/creating-a-cross-section-xml-file/3451 "2023-09-21T16:17:12Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![LilianaQuintero](https://avatars.discourse-cdn.com/v4/letter/l/b5e925/32.png) [@LilianaQuintero](https://openmc.discourse.group/u/LilianaQuintero)\
**Post date:** [September 21, 2023, 4:17pm UTC](https://openmc.discourse.group/t/creating-a-cross-section-xml-file/3451/1 "2023-09-21T16:17:12Z")

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Hi! this is probably an extremely simple question but Im brand new with OpenMC and not a very expert linux user. I followed all the install guide and read through the whole User’s guide, nonetheless I run the first example “Pin-cell” and I get the error that there’s no file containing the cross sections. The example says:

“First, it checks whether Materials.cross\_sections has been set, indicating the path to a cross\_sections.xml file.  
If Materials.cross\_sections isn’t set, it looks for the OPENMC\_CROSS\_SECTIONS environment variable.”

But I don not understand where to get the cross section information from? What is cross\_section.xml suppose to have? what is OPENMC\_CROSS\_SECTIONS?.

Again sorry for the “dumb” question…

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**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\
**Post date:** [September 21, 2023, 5:23pm UTC](https://openmc.discourse.group/t/creating-a-cross-section-xml-file/3451/2 "2023-09-21T17:23:26Z")

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@LilianaQuintero welcome to the community! I know you said you read the whole user’s guide, but nevertheless I’m going to point you to the section on [Data configuration](https://docs.openmc.org/en/latest/usersguide/data.html) which explains this at length. The easiest thing to do is to download and unpack one of the pregenerated nuclear data libraries from [https://openmc.org](https://openmc.org) and then set the `OPENMC_CROSS_SECTIONS` environment variable to the `cross_sections.xml` file from the unpacked archive.