:term:`CFOUR_SPIN_FLIP <CFOUR_SPIN_FLIP (CFOUR)>`
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      Controls whether excitation energy calculations allow for a "spin flip" which changes the :math:`M_s` quantum number. Such calculations have some advantages for biradicals and are currently implemented (together with gradients) for CIS and CIS(D) calculations. Options are OFF and ON.

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

