HD_AVG

> 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