add another example to Howto chunk doc page

doc/src/Howto_chunk.txt

@@ -136,7 +136,9 @@ The "compute chunk/spread/atom"_compute_chunk_spread_atom.html command
 spreads per-chunk values to each atom in the chunk, producing per-atom
 values as its output.  This can be useful for outputting per-chunk
 values to a per-atom "dump file"_dump.html.  Or for using an atom's
-associated chunk value in an "atom-style variable"_variable.html.
+associated chunk value in an "atom-style variable"_variable.html.  Or
+as input to the "fix ave/chunk"_fix_ave_chunk.html command to
+spatially average per-chunk values calculated by a per-chunk compute.
 
 The "compute reduce/chunk"_compute_reduce_chunk.html command reduces a
 peratom value across the atoms in each chunk to produce a value per
@@ -184,12 +186,20 @@ compute cc1 all chunk/atom c_cluster compress yes
 compute size all property/chunk cc1 count
 fix 1 all ave/histo 100 1 100 0 20 20 c_size mode vector ave running beyond ignore file tmp.histo :pre
 
-(6) An example of using a per-chunk value to apply per-atom forces to
+(6) An example for using a per-chunk value to apply per-atom forces to
 compress individual polymer chains (molecules) in a mixture, is
 explained on the "compute
 chunk/spread/atom"_compute_chunk_spread_atom.html command doc page.
 
-(7) An example of using one set of per-chunk values for molecule
+(7) An example for using one set of per-chunk values for molecule
 chunks, to create a 2nd set of micelle-scale chunks (clustered
 molecules, due to hydrophobicity), is explained on the "compute
 chunk/reduce"_compute_reduce_chunk.html command doc page.
+
+(8) An example for using one set of per-chunk values (dipole moment
+vectors) for molecule chunks, spreading the values to each atom in
+each chunk, then defining a second set of chunks as spatial bins, and
+using the "fix ave/chunk"_fix_ave_chunk.html command to calulate an
+average dipole moment vector for each bin.  This example is explained
+on the "compute chunk/spread/atom"_compute_chunk_spread_atom.html
+command doc page.

doc/src/compute_chunk_spread_atom.txt

@@ -30,11 +30,18 @@ compute 1 all chunk/spread/atom mychunk c_com[*] c_gyration :pre
 [Description:]
 
 Define a calculation that "spreads" one or more per-chunk values to
-each atom in the chunk.  This can be useful for creating a "dump
-file"_dump.html where each atom lists info about the chunk it is in,
-e.g. for post-processing purposes.  It can also be used in "atom-style
-variables"_variable.html that need info about the chunk each atom is
-in.  Examples are given below.
+each atom in the chunk.  This can be useful in several scenarios:
+
+For creating a "dump file"_dump.html where each atom lists info about
+the chunk it is in, e.g. for post-processing purposes. :ulb,l
+
+To access chunk value in "atom-style variables"_variable.html that
+need info about the chunk each atom is in. :l
+
+To use the "fix ave/chunk"_fix_ave_chunk.html command to spatially
+average per-chunk values calculated by a per-chunk compute. :l,ule
+
+Examples are given below.
 
 In LAMMPS, chunks are collections of atoms defined by a "compute
 chunk/atom"_compute_chunk_atom.html command, which assigns each atom
@@ -148,6 +155,28 @@ thermo_style    custom step etotal press v_ave :pre
 
 :line
 
+Here is an example for using one set of chunks, defined for molecules,
+to compute the dipole moment vector for each chunk.  E.g. for water
+molecules. Then spreading those values to each atom in each chunk.
+Then defining a second set of chunks based on spatial bins.  And
+finally, using the "fix ave/chunk"_fix_ave_chunk.html command to
+calculate an average dipole moment vector per spatial bin.
+
+compute       cmol all chunk/atom molecule
+compute       dipole all dipole/chunk cmol
+compute       spread all chunk/spread/atom cmol c_dipole\[1\] c_dipole\[2\] c_dipole\[3\]
+compute       cspatial all chunk/atom bin/1d z lower 0.1 units reduced
+fix           ave all ave/chunk 100 10 1000 cspatial c_spread\[*\] :pre
+
+Note that the "fix ave/chunk"_fix_ave_chunk.html command requires
+per-atom values as input.  That is why the compute chunk/spread/atom
+command is used to assign per-chunk values to each atom in the chunk.
+If a molecule straddles bin boundaries, each of its atoms contributes
+in a weighted manner to the average dipole moment of the spatial bin
+it is in.
+
+:line
+
 [Output info:]
 
 This compute calculates a per-atom vector or array, which can be