Added minimal info on downloading and building PACE library

doc/src/Build_extras.rst

@@ -52,6 +52,7 @@ This is the list of packages that may require additional steps.
    * :ref:`USER-MESONT <user-mesont>`
    * :ref:`USER-MOLFILE <user-molfile>`
    * :ref:`USER-NETCDF <user-netcdf>`
+   * :ref:`USER-PACE <user-pace>`
    * :ref:`USER-PLUMED <user-plumed>`
    * :ref:`USER-OMP <user-omp>`
    * :ref:`USER-QMMM <user-qmmm>`
@@ -1247,6 +1248,37 @@ be built for the most part with all major versions of the C++ language.
 
 ----------
 
+.. _user-pace:
+
+USER-PACE package
+-----------------------------
+
+This package requires a library that can be downloaded and built 
+in lib/pace or somewhere else, which must be done before building 
+LAMMPS with this package. 
+
+.. tabs::
+
+   .. tab:: CMake build
+
+      The library download and build will happen automatically when USER-PACE
+      is requested.
+
+   .. tab:: Traditional make
+
+      You can download and build the USER-PACE library
+      in one step from the ``lammps/src`` dir, using these commands, 
+      which invoke the ``lib/pace/Install.py`` script. 
+
+      .. code-block:: bash
+
+         $ make lib-pace                          # print help message
+         $ make lib-pace args="-b"                # download and build the default version in lib/pace
+
+      You should not need to edit the ``lib/pace/Makefile.lammps`` file.
+
+----------
+
 .. _user-plumed:
 
 USER-PLUMED package

doc/src/Packages_details.rst

@@ -1378,6 +1378,10 @@ Aidan Thompson^3, Gabor Csanyi^2, Christoph Ortner^4, Ralf Drautz^1.
 
  ^4: University of British Columbia, Vancouver, BC, Canada
 
+**Install:**
+
+This package has :ref:`specific installation instructions <user-pace>` on the :doc:`Build extras <Build_extras>` page.
+
 **Supporting info:**
 
 * src/USER-PACE: filenames -> commands