PRECONDITIONER

> 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 H0_BLOCKSIZE 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, ITER_INV, EVANGELISTI
> - Default: DAVIDSON