Provide some additional explanations for the "boost neigh_modify one" error

doc/src/Errors_details.rst

@@ -982,3 +982,50 @@ order of preference there are:
 - Send an email to ``developers@lammps.org``
 - Send an email to an :doc:`individual LAMMPS developer <Intro_authors>`
   that you know and trust
+
+.. _err0036:
+
+Neighbor list overflow, boost neigh_modify one
+----------------------------------------------
+
+The neighbor list code in LAMMPS uses a special memory allocation strategy
+to speed up building and accessing neighbor lists.
+
+Instead of making a memory allocation for each list of neighbors of the atoms
+LAMMPS allocates "pages" that have room for several neighbor lists.  This has
+two main advantages:
+
+#. It is not needed to first count how many neighbors there are for an
+   atom to determine the storage required.  Since the pages are much
+   larger than individual lists, LAMMPS just "fills up" the page until
+   there is not enough space left and then allocates a new page.
+
+#. There are fewer calls to the memory allocator functions (which can be
+   time consuming for long-running jobs and fragmented memory space) and
+   the resulting neighbor lists are close to each other physically which
+   improves cache efficiency.
+
+This is controlled by the two parameters "one" and "page", respectively,
+that can be set via the :doc:`neigh_modify command <neigh_modify>`. The
+parameter "one" is the estimate for the number of entries in a single
+neighbor list. The parameter "page" is the size of the page.  The
+default settings are suitable for most systems.  They need to be changed
+when simulating a system with a very high density or when setting a very
+long cutoff (e.g. :math:`\gtrapprox 15 \AA` with :doc:`units real
+<units>`).  The value of "page" must be at least 10x the value of "one",
+but 50x or 100x are recommended to avoid wasting memory.  The neighbor
+list storage is typically the largest amount of RAM required by a
+LAMMPS calculation.
+
+Even though the LAMMPS error message recommends to increase the "one"
+parameter, this may not be the correct solution since the neighbor list
+overflow can be a symptom of some other error that cannot be easily
+detected.  For example, a frequent reason for an (unexpected) high
+density are incorrect box boundaries or coordinates provided as
+fractional coordinates.  In both cases, LAMMPS cannot easily know
+whether the input geometry has such a high density (and thus requiring
+more neighbor list storage) intentionally.
+
+When boosting (= increasing) the "one" parameter, it is recommended to
+also increase the value for the "page" parameter to maintain the ratio
+between "one" and "page".