Hi Ngan,
I’d guess your 9-layer slab might be non-stoichiometric (not the same stoichiometry as the bulk reference) and that’s where the difference comes from. Surface energies of non-stoichiometric films are quite a bit more complicated (requires some thermodynamic considerations and a reservoir) than for stoichiometric films.
Also, I think you can specify the reference bulk structure in the function .surface_energy(bulk_structure) as well as reference elements for a reservoir.
Best,
Peter
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