Source code for qiskit_nature.second_q.formats.fcidump_translator

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# (C) Copyright IBM 2022, 2023.
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# This code is licensed under the Apache License, Version 2.0. You may
# obtain a copy of this license in the LICENSE.txt file in the root directory
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"""Translator methods for the FCIDump."""

from __future__ import annotations

from qiskit_nature.second_q.hamiltonians import ElectronicEnergy
from qiskit_nature.second_q.problems import ElectronicBasis, ElectronicStructureProblem
from qiskit_nature.second_q.properties import ParticleNumber

from .fcidump import FCIDump


def fcidump_to_problem(fcidump: FCIDump) -> ElectronicStructureProblem:
    """Builds out an :class:`.ElectronicStructureProblem` from a :class:`.FCIDump` instance.

    This method centralizes the construction of an :class:`.ElectronicStructureProblem` from a
    :class:`.FCIDump`.

    Args:
        fcidump: the :class:`.FCIDump` object from which to build the problem.

    Returns:
        An :class:`.ElectronicStructureProblem` instance.
    """

    num_beta = (fcidump.num_electrons - (fcidump.multiplicity - 1)) // 2
    num_alpha = fcidump.num_electrons - num_beta

    particle_number = ParticleNumber(fcidump.num_orbitals)

    electronic_energy = ElectronicEnergy.from_raw_integrals(
        fcidump.hij, fcidump.hijkl, fcidump.hij_b, fcidump.hijkl_bb, fcidump.hijkl_ba
    )
    electronic_energy.nuclear_repulsion_energy = fcidump.constant_energy

    problem = ElectronicStructureProblem(electronic_energy)
    problem.basis = ElectronicBasis.MO
    problem.num_particles = (num_alpha, num_beta)
    problem.num_spatial_orbitals = fcidump.num_orbitals
    problem.properties.particle_number = particle_number

    return problem