update dynamical_matrix docs

doc/src/dynamical_matrix.rst

@@ -40,42 +40,46 @@ Description
 
 Calculate the dynamical matrix by finite difference of the selected group,
 
-.. image:: JPG/dynamical_matrix_dynmat.jpg
-   :align: center
+.. math::
 
-where D is the dynamical matrix and Phi is the force constant matrix defined by
+   D = \frac{\Phi_{ij}^{\alpha\beta}}{\sqrt{M_i M_j}}
 
-.. image:: JPG/dynamical_matrix_force_constant.jpg
-   :align: center
+where D is the dynamical matrix and :math:`\Phi` is the force constant
+matrix defined by
 
-The output for the dynamical matrix is printed three elements at a time. The
-three elements are the three beta elements for a respective i/alpha/j combination. 
-Each line is printed in order of j increasing first, alpha second, and i last.
+.. math::
 
-If the style eskm is selected, the dynamical matrix will be in units of inverse squared
-femtoseconds. These units will then conveniently leave frequencies in THz, where
-frequencies, represented as omega, can be calculated from
+   \Phi_{ij}^{\alpha\beta} = \frac{\partial^2 U}{\partial x_{i,\alpha} \partial x_{j,\beta}}
 
-:c, image(Eqs/dynamical\_matrix\_phonons.jpg)
+   
+The output for the dynamical matrix is printed three elements at a time.
+The three elements are the three :math:`\beta` elements for a respective
+i/:math:`\alpha`/j combination.  Each line is printed in order of j
+increasing first, :math:`\alpha` second, and i last.
+
+If the style eskm is selected, the dynamical matrix will be in units of
+inverse squared femtoseconds. These units will then conveniently leave
+frequencies in THz.
 
 Restrictions
 """"""""""""
 
-
 The command collects an array of nine times the number of atoms in a group
 on every single MPI rank, so the memory requirements can be very significant
 for large systems.
 
 This command is part of the USER-PHONON package.  It is only enabled if
-LAMMPS was built with that package.  See the :doc:`Build package <Build_package>` doc page for more info.
+LAMMPS was built with that package.
+See the :doc:`Build package <Build_package>` doc page for more info.
 
 Related commands
 """"""""""""""""
 
 :doc:`fix phonon <fix_phonon>`
 
-:doc:`compute hma <compute_hma>` uses an analytic formulation of the hessian
-provided by Pair's single\_hessian.
+:doc:`compute hma <compute_hma>` uses an analytic formulation of the
+Hessian provided by a pair_style's Pair::single\_hessian() funcion,
+if implemented.
 
 Default
 """""""