qiskit_addon_sqd.fermion.solve_fermion

solve_fermion(bitstring_matrix, /, hcore, eri, *, open_shell=False, spin_sq=None, max_davidson=100, verbose=None)[source]

Approximate the ground state given molecular integrals and a set of electronic configurations.

Parameters:

  • bitstring_matrix (tuple[ndarray, ndarray] | ndarray) –

    A set of configurations defining the subspace onto which the Hamiltonian will be projected and diagonalized. This is a 2D array of bool representations of bit values such that each row represents a single bitstring. The spin-up configurations should be specified by column indices in range (N, N/2], and the spin-down configurations should be specified by column indices in range (N/2, 0], where N is the number of qubits.

    (DEPRECATED) The configurations may also be specified by a length-2 tuple of sorted 1D arrays containing unsigned integer representations of the determinants. The two lists should represent the spin-up and spin-down orbitals, respectively.

  • hcore (ndarray) – Core Hamiltonian matrix representing single-electron integrals

  • eri (ndarray) – Electronic repulsion integrals representing two-electron integrals

  • open_shell (bool) – A flag specifying whether configurations from the left and right halves of the bitstrings should be kept separate. If False, CI strings from the left and right halves of the bitstrings are combined into a single set of unique configurations and used for both the alpha and beta subspaces.

  • spin_sq (int | None) – Target value for the total spin squared for the ground state. If None, no spin will be imposed.

  • max_davidson (int) – The maximum number of cycles of Davidson’s algorithm

  • verbose (int | None) – A verbosity level between 0 and 10

Returns:

  • Minimum energy from SCI calculation

  • SCI coefficients

  • Average orbital occupancy

  • Expectation value of spin-squared

Return type:

A tuple containing