Try to improve the pair style hybrid docs

This specifically tries to avoid the ambiguous use of "mixing" and
clarify that similar is still different when pair styles are concerned.
See discussion here: https://matsci.org/t/confusion-about-mixing-and-pair-coeff-section/38317/3

doc/src/pair_hybrid.rst

@@ -198,8 +198,8 @@ same:
 
 Coefficients must be defined for each pair of atoms types via the
 :doc:`pair_coeff <pair_coeff>` command as described above, or in the
-data file read by the :doc:`read_data <read_data>` commands, or by
+PairCoeffs or PairIJCoeffs section of the data file read by the
-mixing as described below.
+:doc:`read_data <read_data>` command, or by mixing as described below.
 
 For all of the *hybrid*, *hybrid/overlay*, and *hybrid/scaled* styles,
 every atom type pair I,J (where I <= J) must be assigned to at least one
@@ -208,14 +208,21 @@ examples above, or in the data file read by the :doc:`read_data
 <read_data>`, or by mixing as described below.  Also all sub-styles
 must be used at least once in a :doc:`pair_coeff <pair_coeff>` command.
 
+.. note::
+
+   LAMMPS never performs mixing of parameters from different sub-styles,
+   **even** if they use the same type of coefficients, e.g. contain
+   a Lennard-Jones potential variant.  Those parameters must be provided
+   explicitly.
+
 If you want there to be no interactions between a particular pair of
-atom types, you have 3 choices.  You can assign the type pair to some
+atom types, you have 3 choices.  You can assign the pair of atom types
-sub-style and use the :doc:`neigh_modify exclude type <neigh_modify>`
+to some sub-style and use the :doc:`neigh_modify exclude type <neigh_modify>`
 command.  You can assign it to some sub-style and set the coefficients
 so that there is effectively no interaction (e.g. epsilon = 0.0 in a LJ
 potential).  Or, for *hybrid*, *hybrid/overlay*, or *hybrid/scaled*
 simulations, you can use this form of the pair_coeff command in your
-input script:
+input script or the "PairIJ Coeffs" section of your data file:
 
 .. code-block:: LAMMPS
 
@@ -238,19 +245,20 @@ styles with different requirements.
 
 ----------
 
-Different force fields (e.g. CHARMM vs AMBER) may have different rules
+Different force fields (e.g. CHARMM vs. AMBER) may have different rules
-for applying weightings that change the strength of pairwise
+for applying exclusions or wheights that change the strength of pairwise
-interactions between pairs of atoms that are also 1-2, 1-3, and 1-4
+non-bonded interactions between pairs of atoms that are also 1-2, 1-3,
-neighbors in the molecular bond topology, as normally set by the
+and 1-4 neighbors in the molecular bond topology. This is normally a
-:doc:`special_bonds <special_bonds>` command.  Different weights can be
+global setting defined the :doc:`special_bonds <special_bonds>` command.
-assigned to different pair hybrid sub-styles via the :doc:`pair_modify
+However, different weights can be assigned to different hybrid
-special <pair_modify>` command. This allows multiple force fields to be
+sub-styles via the :doc:`pair_modify special <pair_modify>` command.
-used in a model of a hybrid system, however, there is no consistent
+This allows multiple force fields to be used in a model of a hybrid
-approach to determine parameters automatically for the interactions
+system, however, there is no consistent approach to determine parameters
-between the two force fields, this is only recommended when particles
+automatically for the interactions **between** atoms of the two force
+fields, thus this approach this is only recommended when particles
 described by the different force fields do not mix.
 
-Here is an example for mixing CHARMM and AMBER: The global *amber*
+Here is an example for combining CHARMM and AMBER: The global *amber*
 setting sets the 1-4 interactions to non-zero scaling factors and
 then overrides them with 0.0 only for CHARMM:
 
@@ -260,7 +268,7 @@ then overrides them with 0.0 only for CHARMM:
    pair_style hybrid lj/charmm/coul/long 8.0 10.0 lj/cut/coul/long 10.0
    pair_modify pair lj/charmm/coul/long special lj/coul 0.0 0.0 0.0
 
-The this input achieves the same effect:
+This input achieves the same effect:
 
 .. code-block:: LAMMPS
 
@@ -270,9 +278,9 @@ The this input achieves the same effect:
    pair_modify pair lj/cut/coul/long special coul 0.0 0.0 0.83333333
    pair_modify pair lj/charmm/coul/long special lj/coul 0.0 0.0 0.0
 
-Here is an example for mixing Tersoff with OPLS/AA based on
+Here is an example for combining Tersoff with OPLS/AA based on
-a data file that defines bonds for all atoms where for the
+a data file that defines bonds for all atoms where - for the
-Tersoff part of the system the force constants for the bonded
+Tersoff part of the system - the force constants for the bonded
 interactions have been set to 0. Note the global settings are
 effectively *lj/coul 0.0 0.0 0.5* as required for OPLS/AA: