doc updates

doc/src/Build_extras.rst

@@ -37,6 +37,7 @@ This is the list of packages that may require additional steps.
    * :ref:`KOKKOS <kokkos>`
    * :ref:`LATTE <latte>`
    * :ref:`MESSAGE <message>`
+   * :ref:`MLIAP <mliap>`
    * :ref:`MSCG <mscg>`
    * :ref:`OPT <opt>`
    * :ref:`POEMS <poems>`
@@ -732,6 +733,40 @@ be installed on your system.
 
 ----------
 
+.. _mliap:
+
+MLIAP package
+---------------------------
+
+Building the MLIAP package requires also enabling the SNAP package.
+There will be an error message if this requirement is not satisfied.
+Using the *mliappy* model also requires enabling the PYTHON package
+**and** requires you have the `cython <https://ctyhon.org>`_ software
+installed and a working ``cythonize`` command.  This requires Python
+version 3.6 or later.
+
+.. tabs::
+
+   .. tab:: CMake build
+
+      .. code-block:: bash
+
+         -D MLIAP_ENABLE_PYTHON=value   # enable mliappy model (default is autodetect)
+
+      Without this setting, CMake will check whether it can find a
+      suitable Python version and the ``cythonize`` command and choose
+      the default accordingly.
+
+   .. tab:: Traditional make
+
+      The build uses the ``lib/python/Makefile.mliap_python`` file in the
+      compile/link process to add a rule to update the files generated by
+      the ``cythonize`` command in case the corresponding .pyx file was
+      modified.  You may need to need to modify ``lib/python/Makefile.lammps``
+      if the LAMMPS build fails.
+
+----------
+
 .. _mscg:
 
 MSCG package

doc/src/compute_mliap.rst

@@ -56,14 +56,14 @@ and it is also straightforward to add new descriptor styles.
 The compute *mliap* command must be followed by two keywords
 *model* and *descriptor* in either order.
 
-The *model* keyword is followed by the model style (*linear*, *quadratic* or *mliappy*). 
+The *model* keyword is followed by the model style (*linear*, *quadratic* or *mliappy*).
 The *mliappy* model is only available
 if lammps is built with MLIAPPY package.
 
 The *descriptor* keyword is followed by a descriptor style, and additional arguments.
-The compute currently supports just one descriptor style, but it is 
+The compute currently supports just one descriptor style, but it is
 is straightforward to add new descriptor styles.
-The SNAP descriptor style *sna* is the same as that used by :doc:`pair_style snap <pair_snap>`, 
+The SNAP descriptor style *sna* is the same as that used by :doc:`pair_style snap <pair_snap>`,
 including the linear, quadratic, and chem variants.
 A single additional argument specifies the descriptor filename
 containing the parameters and setting used by the SNAP descriptor.

doc/src/pair_mliap.rst

@@ -41,9 +41,9 @@ and the geometric quantities that characterize the atomic positions
 (*descriptor*). By defining *model* and *descriptor* separately,
 it is possible to use many different models with a given descriptor,
 or many different descriptors with a given model. The
-pair style currently supports just one descriptor style, but it is 
+pair style currently supports just one descriptor style, but it is
 is straightforward to add new descriptor styles.
-The SNAP descriptor style *sna* is the same as that used by :doc:`pair_style snap <pair_snap>`, 
+The SNAP descriptor style *sna* is the same as that used by :doc:`pair_style snap <pair_snap>`,
 including the linear, quadratic, and chem variants.
 The available models are *linear*, *quadratic*, and *mliappy*.
 The *mliappy* style can be used to couple python models,
@@ -62,7 +62,7 @@ that specify the mapping of MLIAP
 element names to LAMMPS atom types,
 where N is the number of LAMMPS atom types.
 
-The *model* keyword is followed by the  model style. This is followed 
+The *model* keyword is followed by the  model style. This is followed
 by a single argument specifying the model filename containing the
 parameters for a set of elements.
 The model filename usually ends in the *.mliap.model* extension.