:term:`PRECONDITIONER <PRECONDITIONER (DETCI)>`
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      This specifies the type of preconditioner to use in the selected diagonalization method. The valid options are: ``DAVIDSON`` which approximates the Hamiltonian matrix by the diagonal elements; ``H0BLOCK_INV`` which uses an exact Hamiltonian of :term:`H0_BLOCKSIZE <H0_BLOCKSIZE (DETCI)>` and explicitly inverts it; ``GEN_DAVIDSON`` which does a spectral decomposition of H0BLOCK; ``ITER_INV`` using an iterative approach to obtain the correction vector of H0BLOCK. The ``H0BLOCK_INV``, ``GEN_DAVIDSON``, and ``ITER_INV`` approaches are all formally equivalent but the ``ITER_INV`` is less computationally expensive. Default is ``DAVIDSON``.

      * **Type**: string
      * **Possible Values**: LANCZOS, DAVIDSON, GEN\_DAVIDSON, H0BLOCK, H0BLOCK\_INV, ITER\_INV, H0BLOCK\_COUPLING, EVANGELISTI
      * **Default**: DAVIDSON

