NonBonded LennardJones schema
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A full description of the overall NonBonded LennardJones model.

Required Keys
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The following properties are required for the NonBonded LennardJones specification.
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
   | Key Name                                                                                        | Description                                                                                                              | Field Type                                                                       |
   +=================================================================================================+==========================================================================================================================+==================================================================================+
   | epsilon                                                                                         | The epsilon (well depth) Lennard-Jones parameter. Default unit is kJ/mol.                                                | array[number]                                                                    |
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
   | sigma                                                                                           | The distance at which the Lennard-Jones potential is 0. Default unit is angstroms.                                       | array[number]                                                                    |
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+

Optional Keys
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The following keys are optional for the NonBonded LennardJones specification.
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
   | Key Name                                                                                        | Description                                                                                                              | Field Type                                                                       |
   +=================================================================================================+==========================================================================================================================+==================================================================================+
   | epsilon_units                                                                                   | Units for the Lennard-Jones epsilon (well depth) constant.                                                               | string                                                                           |
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+
   | sigma_units                                                                                     | Units for the Lennard-Jones sigma constant.                                                                              | string                                                                           |
   +-------------------------------------------------------------------------------------------------+--------------------------------------------------------------------------------------------------------------------------+----------------------------------------------------------------------------------+