# DIAG_METHOD

> 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 CI vectors on disk, where is the maximum number of iterations and is the number of roots.

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