Simplify Hamiltonian functions in Molecule class

[chmwzc]

Previously:

Needed three lines to get the molecular Hamiltonian as a SymbolicHamiltonian:

fermionic_ham = mol.fermionic_hamiltonian()
qubit_ham = mol.qubit_hamiltonian(fermionic_ham)
symbolic_ham = mol.symbolic_hamiltonian(qubit_ham)

Gets really tedious after a while...

Now:

Do everything in a single function call instead!

symbolic_ham = mol.hamiltonian()

Another example: HF embedding:

# Run HF embedding
mol.hf_embedding(active=<Your active space here>)
# Define the OEI/TEI to be used for the molecular Hamiltonian
hamiltonian = mol.hamiltonian(oei=mol.embed_oei, tei=mol.embed_tei, constant=mol.inactive_energy)

Notes

• Default arguments of the new hamiltonian() function uses the Molecule class attributes oei, tei, and e_nuc to get the molecular Hamiltonian as a SymbolicHamiltonian directly; no longer need to type so many lines 😄
• If one wants to work with the OpenFermion FermionOperator or QubitOperator, the ham_type argument can be used, with "f" to get a FermionOperator, or "q" to get the QubitOperator.
  • E.g. fermion_operator = mol.hamiltonian("f") returns the molecular Hamiltonian as a FermionOperator
• Arguments for the functions in the previous version of the code are all included as well (see docstring below)

    def hamiltonian(
        self,
        ham_type=None,
        oei=None,
        tei=None,
        constant=None,
        ferm_qubit_map=None,
    ):
        """
        Builds a molecular Hamiltonian using the one-/two- electron integrals

        Args:
            ham_type: Format of molecular Hamiltonian returned
                ("f", "ferm"): OpenFermion FermionOperator
                ("q", "qubit"): OpenFermion QubitOperator
                ("s", "sym"): Qibo SymbolicHamiltonian (default)
            oei: 1-electron integrals. Default: self.oei (MO basis)
            tei: 2-electron integrals in 2ndQ notation. Default: self.tei (MO basis)
            constant: For inactive Fock energy if embedding used. Default: 0.0
            ferm_qubit_map: Which fermion to qubit transformation to use.
                Must be either "jw" (default) or "bk"

            Returns:
                Molecular Hamiltonian in the format of choice
        """

• expectation() and eigenvalues() functions were also cleaned up slightly
  • Draft code (commented out) that uses expectation_from_samples() added in as well