08-Nature Tutorial - QCSchema

#This code is at:
#https://qiskit-community.github.io/qiskit-nature/tutorials/08_qcschema.html

from qiskit_nature.second_q.drivers import PySCFDriver

driver = PySCFDriver()

problem = driver.run()
print(problem)

<qiskit_nature.second_q.problems.electronic_structure_problem.ElectronicStructureProblem object at 0x7aec4135ddd0>

from qiskit_nature.second_q.problems import ElectronicBasis

driver.run_pyscf()
problem = driver.to_problem(basis=ElectronicBasis.MO, include_dipole=True)
print(problem.basis)

ElectronicBasis.MO

ao_problem = driver.to_problem(basis=ElectronicBasis.AO)
print(ao_problem.basis)

The provided alpha-beta overlap matrix is NOT unitary! This can happen when the alpha- and beta-spin orbitals do not span the same space. To provide an example of what this means, consider an active space chosen from unrestricted-spin orbitals. Computing <S^2> within this active space may not result in the same <S^2> value as obtained on the single-reference starting point. More importantly, this implies that the inactive subspace will account for the difference between these two <S^2> values, possibly resulting in significant spin contamination in both subspaces. You should verify whether this is intentional/acceptable or whether your choice of active space can be improved. As a reference, here is the summed-absolute deviation of `S^T @ S` from the identity: 7.064215023495638

ElectronicBasis.AO

from qiskit_nature.second_q.formats.qcschema_translator import qcschema_to_problem

qcschema = driver.to_qcschema()
ao_problem = qcschema_to_problem(qcschema, basis=ElectronicBasis.AO)

The provided alpha-beta overlap matrix is NOT unitary! This can happen when the alpha- and beta-spin orbitals do not span the same space. To provide an example of what this means, consider an active space chosen from unrestricted-spin orbitals. Computing <S^2> within this active space may not result in the same <S^2> value as obtained on the single-reference starting point. More importantly, this implies that the inactive subspace will account for the difference between these two <S^2> values, possibly resulting in significant spin contamination in both subspaces. You should verify whether this is intentional/acceptable or whether your choice of active space can be improved. As a reference, here is the summed-absolute deviation of `S^T @ S` from the identity: 7.064215023495638

from qiskit_nature.second_q.formats.qcschema_translator import get_ao_to_mo_from_qcschema

basis_transformer = get_ao_to_mo_from_qcschema(qcschema)

mo_problem = basis_transformer.transform(ao_problem)
print(mo_problem.basis)

ElectronicBasis.MO