Enable fully occupied spin registers in UCC-like ansatze

[zpy2001]

Environment

• Qiskit Nature version:0.72
• Python version: 3.9.18
• Operating system: MacOS Sonoma

What is happening?

When I use QEOM, I set the driver as

driver = PySCFDriver(
    atom="H 0 0 0; H 0 0 0.7",
    basis="sto3g",
    charge=-1,
    spin=1,
    unit=DistanceUnit.ANGSTROM,
)

It is right in pyscf:

but meets bug in ansatz:

How can we reproduce the issue?

In docs/tutorials/04_excited_states_solvers.ipynb, input the driver as data above.

What should happen?

It throw out an error
ValueError: The number of spatial orbitals 2must be greater than number of particles of any spin kind (2, 1).

Any suggestions?

No response

[mrossinek]

UCC (and its variants) is currently hard-coding a check to ensure that no spin register is completely occupied:

qiskit-nature/qiskit_nature/second_q/circuit/library/ansatzes/ucc.py

Line 394 in cbba76c

if any(n > self.num_spatial_orbitals for n in self.num_particles):

I think that the code itself should be able to handle the case where the number of particles in a spin exactly fills that spin register but there are no unittests to ensure this. Lifting this limitation will enable this use case to run.

---

You can already use generalized=True which will ignore the actual occupation that you start from to run the code. But your ansatz will be longer.