next set of converted AtC documentation

doc/src/atc_source.rst

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+.. index:: fix_modify AtC source
+
+fix_modify AtC source command
+==============================
+
+Syntax
+""""""
+
+.. parsed-literal::
+
+   fix_modify <AtC fixID> source <field> <element_set> <value|function>
+
+* AtC fixID = ID of :doc:`fix atc <fix_atc>` instance
+* source = name of the AtC sub-command
+* field = field kind name valid for type of physics: *temperature* or *electron_temperature*
+* element_set = name of set of elements
+* *value* or *function* = value or name of function followed by its parameters
+
+Examples
+""""""""
+
+.. code-block:: LAMMPS
+
+   fix_modify AtC source temperature middle temporal_ramp 10.0 0.0
+
+Description
+"""""""""""
+
+Add domain sources to the mesh. The units are consistent with LAMMPS's
+units for mass, length and time and are defined by the PDE being solved,
+e.g. for thermal transfer the balance equation is for energy and source
+is energy per time.
+
+Restrictions
+""""""""""""
+
+The keyword *all* is reserved and thus not available as element_set name.
+
+Related AtC commands
+""""""""""""""""""""
+
+:doc:`fix_modify AtC remove_source <atc_remove_source>`
+
+Default
+"""""""
+
+None.