From cf96ddf8bbedac4d72e1730e313d5b7a9cd94284 Mon Sep 17 00:00:00 2001
From: Jose Ignacio Marquez Damian <22483345+marquezj@users.noreply.github.com>
Date: Thu, 17 Nov 2022 13:47:03 +0100
Subject: [PATCH] Basic introduction to NCrystal

Included references to the main papers and documentation
---
 docs/source/methods/cross_sections.rst | 25 +++++++++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/docs/source/methods/cross_sections.rst b/docs/source/methods/cross_sections.rst
index 3396d7f25..c700b3f2d 100644
--- a/docs/source/methods/cross_sections.rst
+++ b/docs/source/methods/cross_sections.rst
@@ -178,6 +178,24 @@ been selected. There are three methods available:
             section data is loaded for a single temperature and is used in the
             unresolved resonance and fast energy ranges.
 
+------------------
+NCrystal materials
+------------------
+
+As an alternative of the standard thermal scattering treatment using :math:`S(\alpha,\beta)`
+tables, OpenMC allows to create materials using NCrystal_. In addition to the regular thermal elastic,
+and thermal inelastic processes, NCrystal allows the generation of models for materials that 
+cannot currently included in ACE files such as oriented single crystals (see the `NCrystal paper`_),
+and further extend the physics `using plugins`_. Thermal scattering kernels are generated on
+the fly from dynamic and structural data, or loaded from :math:`S(\alpha,\beta)` tables converted
+from ENDF6 evaluations. These kernels are sampled in a direct way using a fast `rejection algorithm`_ 
+that does not require previous processing. A `large library` of materials is already included in
+the NCrystal distribution, and new materials can be easily defined from scratch in the `NCMAT format`
+or `combining existing files`.
+
+The compositions of the materials defined in NCrystal are passed on to OpenMC all other reactions 
+except for thermal neutron scattering are handled by continuous energy ACE libraries.
+
 ----------------
 Multi-Group Data
 ----------------
@@ -279,3 +297,10 @@ or even isotropic scattering.
 .. _ENDF/B data: https://www.nndc.bnl.gov/endf-b8.0/
 .. _Leppanen: https://doi.org/10.1016/j.anucene.2009.03.019
 .. _algorithms: http://ab-initio.mit.edu/wiki/index.php/Faddeeva_Package
+.. _NCrystal: https://github.com/mctools/ncrystal
+.. _NCrystal paper: https://doi.org/10.1016/j.cpc.2019.07.015
+.. _using plugins: https://doi.org/10.1016/j.cpc.2021.108082
+.. _rejection algorithm: https://doi.org/10.1016/j.jcp.2018.11.043
+.. _large library: https://github.com/mctools/ncrystal/wiki/Data-library
+.. _NCMAT format: https://github.com/mctools/ncrystal/wiki/NCMAT-format
+.. _combining existing files: https://github.com/mctools/ncrystal/wiki/Announcement-Release3.0.0#2-multiphase-materials