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Unitary Coupled Cluster (UCC) Ansatz

The Unitary Coupled Cluster (UCC) is a commonly used chemistry-inspired ansatz, which is a unitary version of the classical coupled cluster (CC) method [1] .

Syntax

UCCParameters:

  • excitations: List[int] – List of the desired excitations.

Allowed excitations:

  • 1 for singles

  • 2 for doubles

  • 3 for triples

  • 4 for quadruples

Example

First the quantum program is initialized to the Hartree-Fock state, then the UCC function is applied to generate the desired excitations. Here they are single and double, making it a UCCSD ansatz.

from classiq import construct_chemistry_model, synthesize
from classiq.applications.chemistry import Molecule, MoleculeProblem
from classiq.applications.chemistry import UCCParameters, ChemistryExecutionParameters
from classiq.execution import OptimizerType

from classiq import execute

molecule = Molecule(
    atoms=[("H", (0.0, 0.0, 0.0)), ("H", (0.0, 0.0, 0.735))],
)
gs_problem = MoleculeProblem(
    molecule=molecule,
    mapping="jordan_wigner",
)

ansatz_parameters = UCCParameters(
    excitations=[1, 2],
)
execution_params = ChemistryExecutionParameters(
    optimizer=OptimizerType.COBYLA,
    max_iteration=30,
)

model = construct_chemistry_model(
    chemistry_problem=gs_problem,
    use_hartree_fock=True,
    ansatz_parameters=ansatz_parameters,
    execution_parameters=execution_params,
)
qprog = synthesize(model)

result = execute(qprog).result()

The output quantum program:

alt text

[1] Panagiotis Kl. Barkoutsos, Jerome F. Gonthier, Igor Sokolov, Nikolaj Moll, Gian Salis, Andreas Fuhrer, Marc Ganzhorn, Daniel J. Egger, Matthias Troyer, Antonio Mezzacapo, Stefan Filipp, and Ivano Tavernelli Quantum algorithms for electronic structure calculations: Particle-hole Hamiltonian and optimized wave-function expansions Phys. Rev. A 98, 022322 (2018).