Wrong Oriented Unit Cell when Calling SlabGenerator.get_slab

[ThomasWarford]

I will put in a PR for this.

Python version

Python 3.11.0

Pymatgen version

Version: 2025.10.7 (current master branch)

Operating system version

No response

Current behavior

For MgO(111): a and b differ for the orientated unit cell and slab. The number of atoms per layer in the OUC is 4 whilst for the slab it is 1.

This is expected from looking at the logic here, but I think it isn't what we want.

pymatgen/src/pymatgen/core/surface.py

Line 1179 in cc57da9

ouc = ouc if (slab_l.a == ouc.lattice.a and slab_l.b == ouc.lattice.b) else self.oriented_unit_cell

Expected Behavior

I believe the a and b lattice vectors of the OUC should be identical to those of the slab, with c in the same direction as the c vector of the slab. I might be mistaken here.

pymatgen/src/pymatgen/core/surface.py

Line 1179 in cc57da9

ouc = ouc if (slab_l.a == ouc.lattice.a and slab_l.b == ouc.lattice.b) else self.oriented_unit_cell

Minimal example

from pymatgen.core import Lattice, Structure
from pymatgen.core.surface import SlabGenerator

# Rocksalt MgO conventional cell
bulk = Structure.from_spacegroup(
    "Fm-3m",
    Lattice.cubic(4.194),
    ["Mg", "O"],
    [[0, 0, 0], [0.5, 0.5, 0.5]],
)

gen = SlabGenerator(
    initial_structure=bulk,
    miller_index=(1, 1, 1),
    min_slab_size=10,
    min_vacuum_size=20,
    primitive=True,
    lll_reduce=False,
    center_slab=False,
    max_normal_search=None,
)

slab = gen.get_slabs()[0]

sl = slab.lattice
ol = slab.oriented_unit_cell.lattice

print("slab a,b,gamma:", sl.a, sl.b, sl.gamma, sl.alpha, sl.beta)
print("ouc  a,b,gamma:", ol.a, ol.b, ol.gamma, ol.alpha, ol.beta)
print("a/b match:", sl.a == ol.a and sl.b == ol.b)

Relevant files to reproduce this bug

No response