Experiments · E19

Is the screening model equally reliable for every kind of atomic structure?

No. Errors range from 6.51 to 119.50 meV/atom; intermetallic compounds, the key competing phases, are a weak spot at 39.09.

In the log: The accuracy envelope: where MACE-MPA-0 is trustworthy and where it is not

recordedDate 2026-09-12, as written in the logrung 2 · hull, MACE0 predictions · 0 result paragraphsEXPERIMENTS.md lines 825–871
exp E19 diagram
What E19 did and how it came out, drawn from this record and the files it names (book/assets/diagrams/exp/E19.svg).

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The full record

EXPERIMENTS.md · lines 825–871

E19 — The accuracy envelope: where MACE-MPA-0 is trustworthy and where it is not

Date 2026-09-12 · Question An environment is only usable if its error is known per regime, not as one global number. · Provenance …/rhea_envelope.py

Method. 120 random structures from each RHEA config_type, per-element referencing fitted within each group separately, MACE-MPA-0. Unfiltered: each group mixes near- and far-from-equilibrium sampling as RHEA generated it.

config_type n atoms MAE RMSE max
defects 120 126 6.51 9.23 36.4
fcc_alloys 120 32 10.51 13.74 49.9
hcp_alloys 120 48 10.84 14.19 46.5
bcc_alloys_ordered 120 16 13.67 19.95 79.1
bcc_alloys_surface 120 48 13.72 19.09 89.7
fcc_binary_alloys 120 32 13.78 18.55 46.7
liquid 120 128 16.49 25.25 177.6
misc_alloys 120 54 31.44 53.39 287.8
hcp_binary_alloys 120 54 33.16 41.54 118.8
intermetallics 120 24 39.09 52.43 170.7
bcc_alloys 120 54 47.94 69.27 330.8
bcc_binary_alloys 120 54 58.86 85.05 365.2
short_range 120 16 119.50 160.92 581.2

meV/atom.

Two findings that change priorities.

  1. FCC and HCP are among the best regimes (10.5 and 10.8). The lattice is not an obstacle to extending this work to face-centred systems, which is encouraging for nickel superalloys. Magnetism remains untested - RHEA contains no Fe, Co or Ni, so nothing here speaks to ferromagnetic alloys.
  2. Intermetallics are the weak regime at 39.09 meV/atom, and that is precisely where the convex hull lives. Laves, sigma and mu phases are the competing phases that decide whether a solid solution is actually stable (see 4c). So the highest-value fine-tuning target is not the solid solutions but the intermetallics - the opposite of the obvious choice.

short_range at 119.50 is structures with atoms pushed close together to train the repulsive wall; irrelevant to screening and expected to be poor.

Caveat. These are unfiltered. bcc_alloys reads 47.94 here but 6.61 when restricted to within 5% of the Vegard volume (E18), so the ordering partly reflects how aggressively RHEA sampled each group rather than intrinsic difficulty. The same near-equilibrium filter has not been applied group by group.

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