add docs for region2vmd command

doc/src/Commands_all.rst

@@ -140,6 +140,7 @@ additional letter in parenthesis: k = KOKKOS.
    * :doc:`plugin <plugin>`
    * :doc:`prd <prd>`
    * :doc:`python <python>`
+   * :doc:`region2vmd <region2vmd>`
    * :doc:`tad <tad>`
    * :doc:`temper <temper>`
    * :doc:`temper/grem <temper_grem>`

doc/src/commands_list.rst

@@ -82,6 +82,7 @@ Commands
    read_dump
    read_restart
    region
+   region2vmd
    replicate
    rerun
    reset_atoms

doc/src/region2vmd.rst

@@ -0,0 +1,66 @@
+.. index:: region2vmd
+
+region2cmd command
+==================
+
+Syntax
+""""""
+
+.. code-block:: LAMMPS
+
+   region2vmd file args
+
+* file = name of VMD script file to write
+* args = one or more region IDs may be appended
+
+Examples
+""""""""
+
+.. code-block:: LAMMPS
+
+   region2vmd regions.vmd box c1 c2
+
+Description
+"""""""""""
+
+Write a `VMD <https:://ks.uiuc.edu/Research/vmd/>`_ Tcl script file with
+commands that aim to create a visualization of :doc:`LAMMPS regions
+<region>`.  There may be multiple region visualizations stored in a
+single file.  Only a limited amount of region styles and settings are
+currently supported. See **Restrictions** below.
+
+The visualization is implemented by creating a new (and empty) "VMD
+molecule" and then using VMD graphics primitives to represent the region
+in VMD.  Each region will be stored in a separate "VMD molecule" with
+the name "LAMMPS region <region ID>".
+
+The created file can be loaded into VMD either from the command line
+with the -e flag, or from the command prompt with play <script file>, or
+from the File menu via "Load VMD visualization state".
+
+----------
+
+Restrictions
+""""""""""""
+
+This command is part of the EXTRA-COMMAND package.  It is only enabled
+if LAMMPS was built with that package.  See the :doc:`Build package
+<Build_package>` page for more info.
+
+Only the following region styles are currently supported: *block*,
+*cylinder*, *cone*, *sphere*.  For region style *cone* one of the two
+radii must be zero, since the equivalent VMD graphics primitive does not
+support truncated cones.
+
+Moving or rotating regions as well as unions or intersecting regions are
+also currently not supported.
+
+Related commands
+""""""""""""""""
+
+:doc:`region <region>`
+
+Default
+"""""""
+
+none