:term:`HD_AVG <HD_AVG (DETCI)>`
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      How to average H diag energies over spin coupling sets. ``HD_EXACT`` uses the exact diagonal energies which results in expansion vectors which break spin symmetry. ``HD_KAVE`` averages the diagonal energies over a spin-coupling set yielding spin pure expansion vectors. ``ORB_ENER`` employs the sum of orbital energy approximation giving spin pure expansion vectors but usually doubles the number of Davidson iterations. ``EVANGELISTI`` uses the sums and differences of orbital energies with the SCF reference energy to produce spin pure expansion vectors. ``LEININGER`` approximation which subtracts the one-electron contribution from the orbital energies, multiplies by 0.5, and adds the one-electron contribution back in, producing spin pure expansion vectors and developed by Matt Leininger and works as well as ``EVANGELISTI``. 

      * **Type**: string
      * **Possible Values**: EVANGELISTI, HD\_EXACT, HD\_KAVE, ORB\_ENER, LEININGER, Z\_KAVE
      * **Default**: EVANGELISTI