support providing spring constant as equal or atom style variable

doc/src/fix_spring_self.rst

@@ -13,7 +13,7 @@ Syntax
 
 * ID, group-ID are documented in :doc:`fix <fix>` command
 * spring/self = style name of this fix command
-* K = spring constant (force/distance units)
+* K = spring constant (force/distance units), can be a variable (see below)
 * dir = xyz, xy, xz, yz, x, y, or z (optional, default: xyz)
 
 Examples
@@ -22,6 +22,7 @@ Examples
 .. code-block:: LAMMPS
 
    fix tether boundary-atoms spring/self 10.0
+   fix var all spring/self v_kvar
    fix zrest  move spring/self 10.0 z
 
 Description
@@ -42,6 +43,22 @@ directions, but it can be limited to the xy-, xz-, yz-plane and the
 x-, y-, or z-direction, thus restraining the atoms to a line or a
 plane, respectively.
 
+The force constant *k* can be specified as an equal-style or atom-style
+:doc:`variable <variable>`.  If the value is a variable, it should be specified
+as v_name, where name is the variable name.  In this case, the variable
+will be evaluated each time step, and its value(s) will be used as
+force constant for the spring force.
+
+Equal-style variables can specify formulas with various mathematical
+functions and include :doc:`thermo_style <thermo_style>` command
+keywords for the simulation box parameters, time step, and elapsed time.
+Thus, it is easy to specify a time-dependent force field.
+
+Atom-style variables can specify the same formulas as equal-style
+variables but can also include per-atom values, such as atom
+coordinates.  Thus, it is easy to specify a spatially-dependent force
+field with optional time-dependence as well.
+
 Restart, fix_modify, output, run start/stop, minimize info
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