:term:`R4S <R4S (DETCI)>`
"""""""""""""""""""""""""

      Do restrict strings with :math:`e-` in RAS IV? Useful to reduce the number of strings required if MIXED4=true, as in a split-virutal CISD[TQ] computation. If more than one electron is in RAS IV, then the holes in RAS I cannot exceed the number of particles in RAS III + RAS IV (i.e., :term:`EX_LEVEL <EX_LEVEL (DETCI)>` , or else the string is discarded. 

      * **Type**: :ref:`boolean <op_c_boolean>`
      * **Default**: false

