A one-file change to `qiskit_addon_slc/utils/davidson.py` would make Windows installs work

[Mostafa-Atallah2020]

Environment

• qiskit-addon-slc version: 0.1.0
• Python version: 3.12.10
• Operating system: Windows 11

What is happening and why is it wrong?

qiskit-addon-slc is marked Linux/macOS only because of one dependency, pyscf>=2.10,<3, which has no Windows wheel on PyPI or conda-forge and whose source build is broken on Windows (CMake FindBLAS failure, and further failures past that on bzip2, a missing libxcfun.patch, and 'true' is not recognized as an internal or external command). Every other direct dep already ships a Windows wheel.

But pyscf is barely used here. A repo-wide grep finds it in two files: pyproject.toml (the declaration) and qiskit_addon_slc/utils/davidson.py, which has one import pyscf and one call to pyscf.lib.davidson1, a generic sparse-matrix Davidson eigensolver. That helper has one caller in the package (qiskit_addon_slc/bounds/forward.py:187). Replacing that one call with a scipy equivalent (already a transitive dep via qiskit) drops pyscf entirely and lets qiskit-addon-slc install cleanly on Windows alongside Linux and macOS.

How can we reproduce the issue?

py -3.12 -m venv venv
.\venv\Scripts\Activate.ps1
pip install qiskit-addon-slc

Traceback

Captured with MSVC Build Tools 2022 + CMake 3.29 already installed (same failure on a clean machine too). The from-source pyscf build failure:

Collecting pyscf<3,>=2.10 (from qiskit-addon-slc)
  ...
  Building wheel for pyscf (pyproject.toml): started
  Building wheel for pyscf (pyproject.toml): finished with status 'error'
  error: subprocess-exited-with-error

  × Building wheel for pyscf (pyproject.toml) did not run successfully.
  │ exit code: 1
  ╰─> [output]
      ...
      -- Looking for sgemm_ - not found
      ...
      -- Could NOT find BLAS (missing: BLAS_LIBRARIES)
      CMake Error at CMakeLists.txt:79 (message):
        A required library with BLAS API not found.

      -- Configuring incomplete, errors occurred!
      error: command 'cmake.EXE' failed with exit code 1

ERROR: Failed building wheel for pyscf
Failed to build pyscf
ERROR: Failed to build installable wheels for some pyproject.toml based projects (pyscf)

The --no-deps import failure, which exposes where the actual coupling lives:

File ".../qiskit_addon_slc/bounds/__init__.py", line 35, in <module>
    from .backward import compute_backward_bounds
File ".../qiskit_addon_slc/bounds/backward.py", line 36, in <module>
    from ..utils import remove_measure
File ".../qiskit_addon_slc/utils/__init__.py", line 44, in <module>
    from .davidson import get_extremal_eigenvalue
File ".../qiskit_addon_slc/utils/davidson.py", line 22, in <module>
    import pyscf
ModuleNotFoundError: No module named 'pyscf'

Any suggestions?

Replace pyscf.lib.davidson1 with scipy.sparse.linalg.lobpcg in qiskit_addon_slc/utils/davidson.py. It is a drop-in: same problem (smallest eigenvalue of a sparse Hermitian matrix with a diagonal preconditioner), same shape of API (callable A @ v, initial guess x0, preconditioner, tolerance, max iterations). scipy is already a transitive dep of qiskit, so no new requirement is introduced. The signature of get_extremal_eigenvalue stays unchanged, so bounds/forward.py:187 does not change at all. After this, pyscf can be removed from pyproject.toml entirely, the Linux/macOS-only badge and classifiers can be dropped, and Windows users can pip install qiskit-addon-slc like everyone else.

[Mostafa-Atallah2020]

I am asking to link a PR to this issue if it sounds resonable.