Add docs for mdi_engine command and fix

doc/src/Build_extras.rst

@@ -49,6 +49,7 @@ This is the list of packages that may require additional steps.
    * :ref:`USER-COLVARS <user-colvars>`
    * :ref:`USER-H5MD <user-h5md>`
    * :ref:`USER-INTEL <user-intel>`
+   * :ref:`USER-MDI <user-mdi>`
    * :ref:`USER-MESONT <user-mesont>`
    * :ref:`USER-MOLFILE <user-molfile>`
    * :ref:`USER-NETCDF <user-netcdf>`
@@ -1539,6 +1540,35 @@ TBB and MKL.
 
 ----------
 
+.. _user-mdi:
+
+USER-MDI package
+-----------------------------
+
+.. tabs::
+
+   .. tab:: CMake build
+
+      .. code-block:: bash
+
+         -D DOWNLOAD_MDI=value    # download MDI Library for build, value = no (default) or yes
+
+   .. tab:: Traditional make
+
+      Before building LAMMPS, you must build the MDI Library in
+      ``lib/mdi``\ .  You can do this by executing a command like one
+      of the following from the ``lib/mdi`` directory:
+
+      .. code-block:: bash
+
+         $ python Install.py -m gcc       # build using gcc compiler
+         $ python Install.py -m icc       # build using icc compiler
+
+      The build should produce two files: ``lib/mdi/includelink/mdi.h``
+      and ``lib/mdi/liblink/libmdi.so``\ .
+
+----------
+
 .. _user-mesont:
 
 USER-MESONT package

doc/src/fix_mdi_engine.rst

@@ -0,0 +1,58 @@
+.. index:: fix move
+
+fix mdi/engine command
+================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   fix ID group-ID mdi/engine
+
+* ID, group-ID are documented in :doc:`fix <fix>` command
+* mdi/engine = style name of this fix command
+
+Examples
+""""""""
+
+.. code-block:: LAMMPS
+
+   fix 1 all mdi/engine
+
+Description
+"""""""""""
+
+This fix is used alongside the :doc:`mdi_engine <mdi_engine>` command
+to enable LAMMPS to use the
+`MDI Library <https://molssi-mdi.github.io/MDI_Library/html/index.html>`_
+to run as an MDI engine.
+The fix provides hooks that are enable external drivers to communicate
+with LAMMPS at various points within LAMMPS timesteps.
+
+It is not generally necessary to add this fix to a LAMMPS input file,
+even when using the :doc:`mdi_engine <mdi_engine>` command; if the
+:doc:`mdi_engine <mdi_engine>` command is executed and this fix is not
+present, it will automatically be added to the end of the fix list and
+applied to all atoms for the duration of the command.  It is only
+necessary to add this fix to an input file for cases in which the user
+would like to modify order or group-ID of the fix.
+
+For more information about running LAMMPS as an MDI engine, see the
+:doc:`mdi_engine <mdi_engine>` command.
+
+
+Restrictions
+""""""""""""
+This command is part of the USER-MDI package.  It is only enabled if
+LAMMPS was built with that package.  See the :doc:`Build package <Build_package>` doc page for more info.
+
+Related commands
+""""""""""""""""
+
+:doc:`mdi_engine <mdi_engine>`
+
+Default
+"""""""
+
+none

doc/src/mdi_engine.rst

@@ -0,0 +1,45 @@
+.. index:: mdi_engine
+
+mdi_engine command
+===============
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   mdi_engine
+
+Description
+"""""""""""
+
+This command causes LAMMPS to use the
+`MDI Library <https://molssi-mdi.github.io/MDI_Library/html/index.html>`_
+to run as an MDI Engine, responding to commands from an external
+MDI Driver.
+General information about launching codes that communicate using the
+MDI Library can be found in the
+`corresponding page <https://molssi-mdi.github.io/MDI_Library/html/library_page.html#library_launching_sec>`_
+of the MDI Library's documentation.
+
+----------
+
+Other commands can be executed both before and after this command,
+but typically this command should be used at the end of a LAMMPS
+input script.
+
+Restrictions
+""""""""""""
+
+This command is part of the USER-MDI package.  It is only enabled if
+LAMMPS was built with that package.  See the :doc:`Build package <Build_package>` doc page for more info.
+
+Related commands
+""""""""""""""""
+
+:doc:`fix mdi/engine <fix_mdi_engine>`
+
+Default
+"""""""
+
+none

examples/USER/mdi/README

@@ -3,10 +3,6 @@ MDI engine.  LAMMPS as an engine performs the MD timestepping.
 The driver is a simple Python script.  Every timestep the driver
 sends one or more commands to LAMMPS.
 
-See the MDI package documentation Build_extras.html#mdi and
-Build_extras.html#mdi for more details on how the LAMMPS MDI
-implementation works.
-
 --------------
 
 NOTE: the Script.sh file has comands to perform all the