document the use and generation of "DATE:" and "UNITS:" tags

doc/src/bond_write.rst

@@ -47,7 +47,13 @@ The file is written in the format used as input for the
 :doc:`bond_style <bond_style>` *table* option with *keyword* as the
 section name.  Each line written to the file lists an index number
 (1-N), a distance (in distance units), an energy (in energy units),
-and a force (in force units).
+and a force (in force units). In case a new file is created, the first
+line will be a comment with a "DATE:" and "UNITS:" tag with the current
+date and :doc:`units <units>` settings.  For subsequent invocations of
+the bond_write command for the same file, data will be appended and the
+current units settings will be compared to the data from the header, if
+present, and bond_write will refuse to add a table if the units are not
+the same.
 
 Restrictions
 """"""""""""

doc/src/pair_coeff.rst

@@ -86,7 +86,17 @@ Potential files provided with LAMMPS are in the potentials directory
 of the distribution.  For some potentials, such as EAM, other archives
 of suitable files can be found on the Web.  They can be used with
 LAMMPS so long as they are in the format LAMMPS expects, as discussed
-on the individual doc pages.
+on the individual doc pages.  The first line of potential files may
+contain metadata with upper case tags followed their value. These may
+be parsed and used by LAMMPS.  Currently supported are the "DATE:"
+tag and the "UNITS:" tag.  For pair styles that have been programmed
+to support the metadata, the value of the "DATE:" tag is printed to
+the screen and logfile so that the version of a potential file can be
+later identified.  The "UNITS:" tag indicates the :doc:`units <units>`
+setting required for this particular potential file.  If the potential
+file ware created for a different sets of units, LAMMPS will terminate
+with an error.  If the potential file does not contain the tag, no
+check will be made.
 
 When a pair_coeff command using a potential file is specified, LAMMPS
 looks for the potential file in 2 places.  First it looks in the

doc/src/pair_table.rst

@@ -124,11 +124,14 @@ best effect:
 
 ----------
 
-The format of a tabulated file is a series of one or more sections,
-defined as follows (without the parenthesized comments):
+The format of a tabulated file has an (optional) header followed by a
+series of one or more sections, defined as follows (without the
+parenthesized comments). The header must start with a `#` character
+and the DATE: and UNITS: tags will be parsed and used:
 
 .. parsed-literal::
 
+   # DATE: 2020-06-10  UNITS: real  CONTRIBUTOR: ... (header line)
    # Morse potential for Fe   (one or more comment or blank lines)
 
    MORSE_FE                   (keyword is first text on line)

doc/src/pair_write.rst

@@ -34,7 +34,12 @@ the currently defined pair potential.  This is useful for plotting the
 potential function or otherwise debugging its values.  If the file
 already exists, the table of values is appended to the end of the file
 to allow multiple tables of energy and force to be included in one
-file.
+file.  In case a new file is created, the first line will be a comment
+containing a "DATE:" and "UNITS:" tag with the current date and the
+current :doc:`units <units>` setting as argument.  For subsequent
+invocations of the pair_write command, the current units setting is
+compared against the entry in the file, if present, and pair_write
+will refuse to add a table if the units are not the same.
 
 The energy and force values are computed at distances from inner to
 outer for 2 interacting atoms of type itype and jtype, using the