qc2.algorithms.qiskit.vqe
=========================

.. py:module:: qc2.algorithms.qiskit.vqe

.. autoapi-nested-parse::

   Module defining VQE algorithm for Qiskit-Nature.



Classes
-------

.. autoapisummary::

   qc2.algorithms.qiskit.vqe.VQE


Module Contents
---------------

.. py:class:: VQE(qc2data=None, ansatz=None, active_space=None, mapper=None, estimator=None, optimizer=None, reference_state=None, init_params=None, verbose=0)

   Bases: :py:obj:`qc2.algorithms.base.vqe_base.VQEBASE`


   Main class for the VQE algorithm with Qiskit-Nature.

   This class initializes and executes the VQE algorithm using specified
   quantum components like ansatz, optimizer, and estimator.

   .. attribute:: active_space

      Describes the active space for quantum
      simulation. Defaults to ``ActiveSpace((2, 2), 2)``.

      :type: ActiveSpace

   .. attribute:: mapper

      Strategy for fermionic-to-qubit mapping.
      Defaults to :class:`qiskit.JordanWignerMapper`.

      :type: QubitMapper

   .. attribute:: estimator

      Method for estimating the
      expectation value. Defaults to :class:`qiskit.Estimator`

      :type: BaseEstimator

   .. attribute:: optimizer

      Optimization routine for circuit
      variational parameters. Defaults to
      :class:`qiskit_algorithms.SLSQP`.

      :type: qiskit.Optmizer

   .. attribute:: reference_state

      Reference state for the VQE
      algorithm. Defaults to :class:`qiskit.HartreeFock`.

      :type: QuantumCircuit

   .. attribute:: ansatz

      The ansatz for the VQE algorithm.
      Defaults to :class:`qiskit.UCCSD`.

      :type: UCC

   .. attribute:: params

      List of initial VQE circuit parameters.
      Defaults to a list with entries of zero.

      :type: List

   .. attribute:: verbose

      Verbosity level. Defaults to 0.

      :type: int


   .. py:attribute:: active_space


   .. py:attribute:: mapper


   .. py:attribute:: estimator


   .. py:attribute:: optimizer


   .. py:attribute:: reference_state


   .. py:attribute:: ansatz


   .. py:attribute:: params


   .. py:attribute:: verbose
      :value: 0



   .. py:method:: _get_default_reference(active_space: qc2.algorithms.utils.active_space.ActiveSpace, mapper: qiskit_nature.second_q.mappers.QubitMapper) -> qiskit.circuit.QuantumCircuit
      :staticmethod:


      Set up the default reference state circuit based on Hartree Fock.

      :param active_space: description of the active space.
      :type active_space: ActiveSpace
      :param mapper: mapper class instance.
      :type mapper: mapper

      :returns: Hartree-Fock circuit as the reference state.
      :rtype: QuantumCircuit



   .. py:method:: _get_default_ansatz(active_space: qc2.algorithms.utils.active_space.ActiveSpace, mapper: qiskit_nature.second_q.mappers.QubitMapper, reference_state: qiskit.circuit.QuantumCircuit) -> qiskit_nature.second_q.circuit.library.UCC
      :staticmethod:


      Set up the default UCC ansatz from a Hartree Fock reference state.

      :param active_space: Description of the active space.
      :type active_space: ActiveSpace
      :param mapper: Mapper class instance.
      :type mapper: QubitMapper
      :param reference_state: Reference state circuit.
      :type reference_state: QuantumCircuit

      :returns: UCC ansatz quantum circuit.
      :rtype: UCC



   .. py:method:: _get_default_init_params(nparams: List) -> List
      :staticmethod:


      Generates a list of initial circuit parameters for the ansatz.

      :param nparams: Number of parameters in the ansatz.
      :type nparams: int

      :returns: List of initial parameter values (all zeros).
      :rtype: List[float]



   .. py:method:: run(*args, **kwargs) -> qc2.algorithms.algorithms_results.VQEResults

      Executes the VQE algorithm.

      :param \*args: Variable length argument list to be passed to
                     the :class:`qiskit_algorithm.VQE` class.
      :param \*\*kwargs: Arbitrary keyword arguments to be passed to
                         the :class:`qiskit_algorithm.VQE` class.

      :returns:     An instance of :class:`qc2.algorithms.qiskit.vqe.VQEResults`
                    class with all VQE info.
      :rtype: VQEResults

      **Example**

      >>> from ase.build import molecule
      >>> from qc2.ase import PySCF
      >>> from qc2.data import qc2Data
      >>> from qc2.algorithms.qiskit import VQE
      >>> from qc2.algorithms.utils import ActiveSpace
      >>>
      >>> mol = molecule('H2O')
      >>>
      >>> hdf5_file = 'h2o.hdf5'
      >>> qc2data = qc2Data(hdf5_file, mol, schema='qcschema')
      >>> qc2data.molecule.calc = PySCF()
      >>> qc2data.run()
      >>> qc2data.algorithm = VQE(
      ...     active_space=ActiveSpace(
      ...         num_active_electrons=(2, 2),
      ...         num_active_spatial_orbitals=4
      ...     ),
      ...     optimizer=SLSQP(),
      ...     estimator=Estimator(),
      ... )
      >>> results = qc2data.algorithm.run()