Heterobilayer: DFT indirect → GW direct gap?

[Agathyan Barathi]

Hi all,

I am calculating GW band structures of AA′-stacked TMDC bilayers using QE + BerkeleyGW with Wannier interpolation (sig2wan).

For MoSe₂–WSe₂ (AA′): PBE-DFT gives an indirect gap. GW shifts the CBM to K, giving a direct quasiparticle gap

For MoSe₂–MoSe₂ (AA′): The gap character remains the same in DFT and GW.

Is it reasonable that GW gives this result (indirect → direct) in the heterobilayer, or should this be treated as an issue?

I am uploading input/output files for reference and the Band Structure plots (Comparison of DFT/Wannier and GW band structures)

P.S: Key parameters:

• ecutwfc = 120 Ry, nband = 250
• SCF: 20×20×1, NSCF/WFN: 10×10×1

• GW: num_bands = 200

  • Empty bands: 174 (homo), 167 (hetero)
• Same epsilon and sigma inputs
• Path: Γ–M–K–Γ (Wannier-interpolated)

Thank you.

[Zhenglu Li]

Dear Agathyan,

It is reasonable that GW can change the direct/indirect gap nature from DFT.  However, careful convergence is needed, as well as comparison with existing literature.

200 bands for GW convergence of 2D materials appears low, and might give misleading results.  At least few thousands of bands are needed in this case.

Best,

Zhenglu

---

From: Agathyan Barathi <barathi...@gmail.com>
Sent: Thursday, January 29, 2026 7:21 AM
To: BerkeleyGW Help <he...@berkeleygw.org>
Subject: Heterobilayer: DFT indirect → GW direct gap?

 

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[Agathyan Barathi]

Dear Zhenglu,

Thank you very much for your clarification and for emphasising the importance of careful GW convergence in 2D systems.

I understand your concern that a limited number of bands can lead to misleading GW results. In my tests so far, I increased the number of bands from 200 up to 400 while keeping the same dielectric and self-energy parameters, and the nature of the gap (direct vs. indirect) remained unchanged. However, I fully agree that this alone does not establish convergence.

I also wanted to ask for your perspective regarding the comparison with existing literature. For example, in a recent GW-BSE study (https://doi.org/10.48550/arXiv.2509.14200) on bilayer MoS2 using BerkeleyGW (G0W0 with PBE), the authors report converged quasiparticle and optical properties using 200 bands, an 8 Ry dielectric cutoff. They note that these settings are sufficient to reproduce consistent band gaps and spectra.

And, I would appreciate your advice on how to interpret such results when benchmarking my calculations.

Thank you again for your time and guidance.

Best regards,
Agathyan