What is ACE Library?

Hello,
I just find a problem, I hope someone could help me.
I got the error prompt as shown in the figure attached. I do not even know which file is incorrect. Could anyone help me with it?
Thanks!
Xinyan

Hi Xinyan,

OpenMC uses cross section data in the ACE format, the same format that is used for MCNP. It looks like your cross_setions.xml file is configured to use ACE cross sections from the path /opt/mcnp/data/endf70j. Do you have cross section data in that location?

Best,
Paul

Hello Paul,
You are right. I do not even have such directory. Thanks for your suggestions.
Xinyan