reference options

The reference table contains options used to control the Hilbert space used in the calculation and trial function for the projected estimator.

det

type: vector of integers.

Optional. Default: a simple (but potentially not optimal) guess which satisfies the spin and, if provided, symmetry options using the Aufbau principle. In most cases the default (which for molecules typically corresponds to the Hartree–Fock determinant) is sufficient.

Specify the determinant (as a list of indices corresponding to occupied single-particle orbitals) to be used as the reference determinant, which is used in the trial function for calculating the projected energy estimator. Typically this should be the determinant expected to have the greatest overlap with the desired wavefunction.

hilbert_space_det

type: vector of integers.

Optional. Default: set to det.

Specify the determinant (as a list of indices corresponding to occupied single-particle orbitals) used to generate the Hilbert space. Using different determinants to control the Hilbert space and the trial function allows, for example, spin-flip calculations to be performed.

Note

Only relevant if the Hilbert space is not equivalent to the FCI space, i.e. ex_level is smaller than the number of electrons in the system.

ex_level

type: integer.

Optional. Default: set to the number of electrons in the system (i.e. consider all determinants in the FCI space).

Maximum excitation level to consider relative to the determinant given by hilbert_space_det.