:term:`MRCC_NUM_SINGLET_ROOTS <MRCC_NUM_SINGLET_ROOTS (MRCC)>`
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      Number of singlet roots. (Strictly speaking number of of roots with M_s=0 and S is even.) Use this option only with closed shell reference determinant, it must be zero otherwise. This becomes ``nsing`` (option \#2) in fort.56.

      * **Type**: integer
      * **Default**: 1