REVIEW 2 major objections 4 minor 46 references
No band gap, no problem: Defects in InAs using a band-avoiding occupation-constrained density functional theory
T0 review · 2 major / 4 minor · reviewed 2026-07-30 · grok-4.5
Pith's one-line read A constrained occupation scheme lets standard DFT predict defect levels in InAs even when the computed band gap collapses to zero.
desk verdict Practical occupation fix for collapsed-gap defect totals in InAs; internal numerics are careful, external ID of levels is still thin. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
ba-occ-DFT: a Γ-specific non-Aufbau occupation constraint that skips the collapsed conduction-band-edge state and forces occupation of the flat, localized defect state, used together with local-moment countercharge boundary conditions and adequate k-point sampling.
What would settle it
A direct experimental identification of the As-antisite (0/+) and (+/2+) levels in InAs (or a large-supercell hybrid-functional calculation that cleanly resolves the same Γ-point state ordering) that disagrees with the predicted 0.35 eV and 0.12 eV positions would falsify the method.
Extended reading notes
Core claim
Band-avoiding occupation-constrained DFT (ba-occ-DFT) separates band-edge errors from defect energetics: by emptying the spuriously collapsed conduction-band edge at Γ and occupying the localized defect orbital instead, one obtains uncorrupted total energies and therefore rigorous charge-transition levels for defects in InAs despite a zero DFT gap.
Load-bearing premise
The true localized defect orbital really does continue through the zone center above the fake conduction-band edge, so that forcing its occupation yields the physical defect ground-state energy rather than an artifact.
Editorial extensions
If this is right
- Intrinsic primary defects in InAs terminate at stable (1+) charge states and therefore all act as shallow donors, explaining the broad DLTS shoulder seen after irradiation.
- The As antisite remains a low-formation-energy double donor that can appear in as-grown material, analogous to EL2 in GaAs.
- Ordinary PBE plus the occupation constraint and proper charged-supercell boundary conditions can replace hybrid functionals for defect levels in other zero-gap or narrow-gap III–V alloys.
- The same occupation switch is unnecessary for valence-band-edge crossings; the dominant error is over-stabilization of the delocalized conduction edge, not defect delocalization.
Reading between the lines
- The same Γ-point occupation switch should transfer directly to InSb, InAsSb, and related type-II superlattice detector materials where DFT gaps also collapse.
- Once formation energies are trustworthy, multiscale kinetic models of radiation damage evolution (already demonstrated for GaAs) become feasible for InAs-based devices.
- Codes that already support constrained occupations can implement ba-occ-DFT with only a k-point-dependent occupation mask, lowering the barrier to adoption.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript introduces band-avoiding occupation-constrained DFT (ba-occ-DFT): at Γ, non-Aufbau occupations empty the spuriously collapsed conduction-band-edge (CBE) state and occupy the putative localized defect orbital, while off-Γ points use standard Aufbau. Combined with LMCC charged-supercell boundary conditions and PBE, the method is applied to intrinsic point defects in InAs (antisites, vacancies, divacancy, interstitials) in supercells up to 1000 atoms. The authors report converged defect transition levels despite a zero PBE gap, an effective defect band gap bounded by clean defect states, and qualitative consistency with sparse DLTS data (including a shallow-donor interpretation of a broad emission shoulder). HSE06 band structures in small cells are shown not to remove defect-band dispersion or Γ hybridization.
Significance. If the constrained total energies are physically faithful, the work supplies a practical route to defect levels in narrow-gap III–Vs where standard DFT gaps collapse and hybrid+jellium supercell calculations remain costly. The internal convergence campaign (64→1000 atoms, defect-band flattening, Γ-only vs off-Γ and Aufbau vs ba-occ bond-length diagnostics for AsIn) is careful and falsifiable. Predictions that primary displacement defects terminate as shallow donors offer a concrete experimental target. The approach extends a previously benchmarked total-energy/LMCC framework (Si, GaAs) rather than introducing an uncontrolled free parameter for the gap, which is a genuine methodological strength.
major comments (2)
- [Fig. 1(d); AsIn discussion and SM cross-ref] Central hypothesis (Fig. 1d and text): the claim that ba-occ-DFT yields uncorrupted localized-defect ground-state energies rests on inverted CBE/defect character at Γ and on adiabatic equivalence of the constrained KS state to the physical gapped defect. Main-text support is indirect (supercell flattening, avoided-crossing language, As–As bond lengths that track charge only under ba-occ, near-equality of Γ-(2×2×2) and off-Γ formation energies). A direct main-text diagnostic—e.g., real-space localization or projection of the occupied Γ orbital onto bulk Bloch/CBE character under both Aufbau and ba-occ—should be added (or the SM result elevated and quantified). Without it, absolute levels in Fig. 4 carry an unquantified systematic risk if residual hybridization remains.
- [Fig. 4 and surrounding paragraphs] Fig. 4 level diagram: VBE is bounded by InAs(2+/1+) and CBE is placed at the experimental gap, while an “effective defect band gap” is defined from the highest clean defect state. Charge-transition total-energy differences are method-internal; absolute placement relative to the true edges is not. The text should state explicitly which reported numbers are independent of the experimental-gap alignment, quote the numerical uncertainty from the shallow-bound choice of VBE, and avoid language (“rigorous defect level predictions”) that blurs this distinction.
minor comments (4)
- [passim; End Matter heading] Several run-on or missing-space tokens appear in the compiled text (e.g., “extremeinnarrow-gapsemiconductors”, “bandgapproblem”, “END MA TTER”). A full proofreading pass is needed.
- [Figs. 2 and 3] Fig. 2–3 band-structure panels would be clearer with an explicit legend for occupied vs empty states under ba-occ and a horizontal marker for the constrained defect level at Γ.
- [Discussion of experimental comparison] The experimental associations (Salman; Murawski) are appropriately hedged as non-conclusive; consider adding one sentence on what measurement (e.g., stress splitting, annealing kinetics, or ODMR) would discriminate the predicted AsIn second donor near the VBE.
- [End Matter] End Matter: briefly state whether spin-orbit coupling was tested for bulk InAs edges or any defect, even if only to justify its omission for the present total-energy differences.
Circularity Check
No construction-level circularity: defect levels are independent total-energy differences; only light methodological self-citation of LMCC/occ-DFT priors.
-
self citation load bearing
[Intro paragraphs on Si/GaAs validation; methods citing LMCC and finite-defect model]
"Contrary to conventional wisdom concerning the band gap problem and defect levels, we previously demonstrated that a total-energy-based DFT method for charged defects [19] predicted defect levels in silicon (Si) to within 0.1(average)-0.2(max) eV of experiment... The crucial feature was incorporating rigorous Coulomb boundary conditions... via the local moment countercharge (LMCC) approach [20, 21]."
The premise that DFT total energies plus LMCC already separate band-edge error from defect levels, and are therefore trustworthy once occupations are cleaned up, rests on the authors’ own prior Si/GaAs papers rather than an independent external benchmark for InAs. This is ordinary methodological self-citation and does not make the InAs numbers equal to their inputs by construction; it only slightly elevates reliance on an unverified-in-this-work chain. Not scored higher because the ba-occ constraint and the InAs total-energy differences themselves are new and not defined by those citations.
full rationale
The paper’s derivation chain is: (i) PBE supercell total energies with LMCC charged-boundary conditions; (ii) a new Γ-specific non-Aufbau occupation that empties the collapsed CBE and occupies the putative defect orbital (ba-occ-DFT); (iii) charge-transition levels from total-energy differences; (iv) diagram placement with VBE bounded by the computed InAs(2+/1+) and CBE set to the experimental gap only for display. None of these steps defines the output in terms of the input. There is no fit of a free parameter to InAs defect data that is then re-presented as a prediction, no uniqueness theorem imported from the authors, and no renaming of a known empirical pattern. Self-citations (LMCC, Si/GaAs DFT+LMCC benchmarks, occ-DFT machinery) supply the established numerical method and the prior claim that total-energy defect levels can be accurate despite a KS gap error; they do not algebraically force the InAs level positions. Post-hoc associations with Salman/Murawski DLTS features are not used as fit targets. The weakest point in the paper is the physical hypothesis that the constrained Γ occupation recovers the true localized-defect ground state—an assumption/correctness risk, not a circular reduction. Score 1 only for ordinary methodological self-citation that is not load-bearing for the numerical results.
Assumptions & free parameters
free parameters (2)
- HSE06 exact-exchange fraction (0.25) =
0.25 (default)
- Experimental InAs gap used to place CBE in level diagram =
0.42 eV (0 K literature)
assumptions (5)
- domain assumption PBE total energies of localized defects remain accurate to ~0.1–0.2 eV when band-edge occupations are correctly constrained, as previously validated for Si and GaAs with LMCC.
- domain assumption LMCC local-moment countercharge removes finite-size electrostatic errors for charged supercells sufficiently for meV-level convergence.
- ad hoc to paper At Γ the KS state ordering can invert so that the lowest empty (or spuriously occupied) orbital is CBE-like while a higher orbital is the localized defect; non-Aufbau occupation of the latter is the physical ground state.
- domain assumption Discrete Defect Occupation (uniform occupation of each band across the BZ, zero electronic temperature) is the correct baseline for mapping supercell occupations onto isolated-defect charge states.
- domain assumption Large-core In d0 pseudopotential with NLCC is transferable enough for InAs defect energetics versus a d10 small-core PP.
invented entities (2)
-
ba-occ-DFT (band-avoiding occupation-constrained DFT protocol)
-
Effective defect band gap (EDBG) bounded by clean defect levels
Cite this review
Pith. "Pith review of No band gap, no problem: Defects in InAs using a band-avoiding occupation-constrained density functional theory." pith.science (2026). https://pith.science/paper/ZLRQQH2U
@misc{pith2026260727095,
author = {Pith},
title = {Pith review of: No band gap, no problem: Defects in InAs using a band-avoiding occupation-constrained density functional theory},
year = {2026},
howpublished = {\url{https://pith.science/paper/ZLRQQH2U}},
note = {Machine review of arXiv:2607.27095}
}
read the original abstract
Density functional theory (DFT) underestimates the experimental band gap---the infamous band gap problem. As the band gap defines the energy scale of defect levels, this complicates computation of charge transition energies for atomic defects. In the extreme case of narrow-gap semiconductors, the DFT band gap collapses to zero, seemingly precluding quantitative predictions of defect levels. We present a band-avoiding occupation-constrained DFT (ba-occ-DFT) approach that prevents spurious occupation of band-edge states and enables reliable total energy calculations of atomic defects. Application to indium arsenide (InAs) shows that ba-occ-DFT circumvents the band gap problem, separates band-edge errors from defect level calculations, and enables rigorous defect level predictions in a narrow-gap semiconductor despite a zero DFT band gap.
Figures
Reference graph
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