Best practice for calculating E0s (Isolated Atoms) for MACE fine-tuning: Tetrahedron vs. Gaussian Smearing?

[dominicvarghese]

Hi everyone,

I am currently performing multi-head replay fine-tuning using the MACE-MATPES-R2SCAN foundation model. My fine-tuning dataset consists of PBEsol DFT calculations for $\text{KTaO}_3$ (wide band-gap insulator). For my bulk training data (NPT/NVT runs), I used the Tetrahedron method with Blöchl corrections (ISMEAR = -5) as it is generally preferred for insulators.

I am now generating the isolated atom energies (E0s) required for the fine-tuning process. When I attempt to calculate the isolated Oxygen energy using settings consistent with my bulk calculation (ISMEAR = -5), I obtain a positive total energy, which seems unphysical for a bound atom.

Simulation Details (Oxygen Atom):Box: $15 \times 15 \times 15$ Å
K-Points: 4x4x4 (as required by tetrahedron method)

INCAR
ENCUT   = 500       # Matches bulk exactly
PREC    = Accurate
EDIFF   = 1e-8
LREAL   = .FALSE.
ISYM    = 0         # Symmetry off
IBRION  = -1        # Static
NSW     = 0

Tetrahedron (ISMEAR = -5): TOTEN = 0.11612059 eV (Positive/Unphysical?)

With Gaussian smearing,
ISMEAR = 0, SIGMA = 0.20: TOTEN = -0.32510178 eV
ISMEAR = 0, SIGMA = 0.05: TOTEN = -0.09370829 eV

Given that ISMEAR = -5 is generally ill-defined for isolated atom, is it standard practice to switch to Gaussian smearing (ISMEAR = 0) just for the isolated atoms, even if the bulk data uses Tetrahedron? If switching to Gaussian is the correct approach, how should I select the SIGMA to ensure consistency with the MACE model's expectations?

Any advice on the correct protocol for generating these E0s would be greatly appreciated.