qpe_toolbox.hamiltonian.chemistry

Functions

chemistry_hamiltonian(molecule, hf_mode, *[, ...])	Build a molecular electronic Hamiltonian mapped to qubits.
do_pyscf(molecule, hf_mode, do_fci, do_ccsd)	Run PySCF electronic-structure calculations.
make_qubit_hamiltonian(hf, hf_mode, encoding)	Construct a qubit Hamiltonian from a PySCF Hartree-Fock object.
make_fermionic_hamiltonian(ncas, ecore, hpq, hpqrs, ...)	Build a fermionic second-quantized Hamiltonian.
terms_from_openfermion(qubit_operator)	Convert an OpenFermion qubit operator into quimb Hamiltonian terms.

Module Contents

qpe_toolbox.hamiltonian.chemistry.chemistry_hamiltonian(molecule, hf_mode, *, encoding='original', do_fci=False, do_ccsd=False)

Build a molecular electronic Hamiltonian mapped to qubits.

This function performs a PySCF Hartree-Fock calculation, optionally followed by FCI and/or CCSD, constructs the second-quantized fermionic Hamiltonian, and maps it to a qubit Hamiltonian using the Jordan-Wigner transformation.

Parameters:

• molecule (pyscf.gto.Mole) – PySCF molecule object defining geometry and basis.

• hf_mode ({'rhf', 'uhf'}, optional) – Type of Hartree-Fock calculation. Default is 'rhf'.

• encoding ({'original', 'sf', 'df'}, optional) – Encoding of two-body integrals: - 'original': full integrals - 'sf': single-factorized - 'df': double-factorized

• do_fci (bool, optional) – Whether to compute the FCI ground-state energy.

• do_ccsd (bool, optional) – Whether to compute the CCSD energy.

Returns:

Qubit Hamiltonian with additional attributes: norb, nelec, e_hf, and optionally e_fci and e_ccsd.

Return type:

Hamiltonian

qpe_toolbox.hamiltonian.chemistry.do_pyscf(molecule, hf_mode, do_fci, do_ccsd)

Run PySCF electronic-structure calculations.

Performs a Hartree-Fock calculation, and optionally FCI and CCSD, returning the HF object needed to extract molecular integrals.

Parameters:

• molecule (pyscf.gto.Mole) – Molecular system.

• hf_mode ({'rhf', 'uhf'}) – Hartree-Fock flavor.

• do_fci (bool) – Whether to compute FCI energy.

• do_ccsd (bool) – Whether to compute CCSD energy.

Returns:

• e_fci (float or None) – FCI ground-state energy.

• e_ccsd (float or None) – CCSD ground-state energy.

• hf (pyscf.scf.hf.SCF) – Hartree-Fock object containing molecular orbitals and integrals.

qpe_toolbox.hamiltonian.chemistry.make_qubit_hamiltonian(hf, hf_mode, encoding)

Construct a qubit Hamiltonian from a PySCF Hartree-Fock object.

Parameters:

• hf (pyscf.scf.hf.SCF) – Converged HF object.

• hf_mode ({'rhf', 'uhf'}) – Hartree-Fock type.

• encoding ({'original', 'sf', 'df'}) – Integral encoding scheme.

Returns:

Qubit Hamiltonian with constant energy shift stored in e_const.

Return type:

Hamiltonian

qpe_toolbox.hamiltonian.chemistry.make_fermionic_hamiltonian(ncas, ecore, hpq, hpqrs, hf_mode, encoding, *, verbosity=0)

Build a fermionic second-quantized Hamiltonian.

Parameters:

• ncas (int) – Number of active spatial orbitals.

• ecore (float) – Core (constant) energy contribution.

• hpq (ndarray) – One-body integrals.

• hpqrs (ndarray or tuple of ndarray) – Two-body integrals.

• hf_mode ({'rhf', 'uhf'}) – Hartree-Fock type.

• encoding ({'original', 'sf', 'df'}) – Integral compression scheme.

• verbosity (int, optional) – Verbosity level.

Returns:

Fermionic Hamiltonian operator.

Return type:

openfermion.ops.FermionOperator

qpe_toolbox.hamiltonian.chemistry.terms_from_openfermion(qubit_operator)

Convert an OpenFermion qubit operator into quimb Hamiltonian terms.

This function separates the constant (identity) contribution from Pauli-string terms and converts them into the format expected by the Hamiltonian class.

Parameters:

qubit_operator (openfermion.ops.QubitOperator) – Qubit Hamiltonian expressed as a sum of Pauli strings.

Returns:

• term_list (list of tuple) – List of Hamiltonian terms in quimb format: (coefficient, pauli_string, qubits).

• e_const (float) – Constant energy offset corresponding to the identity term.