How to specify the problem

To specify the problem, we follow the Qiskit Nature workflow. We set up the molecule object, specify the driver, and instantiate the ElectronicStructureProblem.

We first require the following modules:

from qiskit_nature.drivers import Molecule
from qiskit_nature.drivers.second_quantization import PySCFDriver
from qiskit_nature.problems.second_quantization import ElectronicStructureProblem

To set up the molecule object, we specify the individual atoms and their positions:

molecule = Molecule(
        ("H", [0.0, 0.0, 0.0]),
        ("H", [0.0, 0.0, 0.735]),
    multiplicity=1, # Multiplicity (2S+1) of the molecule, where S is the total spin angular momentum

We then specify the driver (see PySCFDriver) and load the molecule into the driver:

driver = PySCFDriver.from_molecule(molecule=molecule, basis="sto6g")

Here, the driver is an algorithm class that knows how to calculate the second quantized operators.

Finally, we instantiate the ElectronicStructureProblem, a class which wraps a number of different types of drivers:

problem = ElectronicStructureProblem(driver)