:term:`CFOUR_SPIN_ORBIT <CFOUR_SPIN_ORBIT (CFOUR)>`
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      Experimental Use! ON (=1) requests calculation of one-electron spin-orbit integrals. MEANSO additionally gives a mean-field treatment of the two-electron terms (spin-orbit mean field treatment as described Mol. Phys. 98, 1823-1833 (2000)).

      * **Type**: string
      * **Possible Values**: ON, MEANSO, OFF
      * **Default**: OFF