:term:`DIAG_METHOD <DIAG_METHOD (DETCI)>`
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      This specifies which method is to be used in diagonalizing the Hamiltonian. The valid options are: ``RSP``, to form the entire H matrix and diagonalize using libciomr to obtain all eigenvalues (n.b. requires HUGE memory); ``OLSEN``, to use Olsen's preconditioned inverse subspace method (1990); ``MITRUSHENKOV``, to use a 2x2 Olsen/Davidson method; and ``DAVIDSON`` (or ``SEM``) to use Liu's Simultaneous Expansion Method, which is identical to the Davidson method if only one root is to be found. There also exists a SEM debugging mode, ``SEMTEST``. The ``SEM`` method is the most robust, but it also requires :math:`2NM+1` CI vectors on disk, where :math:`N` is the maximum number of iterations and :math:`M` is the number of roots.

      * **Type**: string
      * **Possible Values**: RSP, OLSEN, MITRUSHENKOV, DAVIDSON, SEM, SEMTEST
      * **Default**: SEM