[QE-users] Problem with computed bandgap

[Lorenzo Paulatto]

Hello,

when studying the similar compoiunf Ce2O2S, I only had good results 
usign the high-precision Ce pseudopotential 
Ce.pbe-spdfn-rrkjus_psl.1.0.0.UPF from the pslibrary that includes 
5s5p6s4d5d4f in valence (22 electrons in valence) and requires a 
ridiculous 200 Ry cutoff to converge, despite being Ultrasoft. The U was 
almost 7eV. You can find more details in this paper and its 
supplementary materials : 
https://pubs.acs.org/doi/abs/10.1021/acs.chemmater.9b00450 (I can send 
the inout files if you want)

hth

On 29/06/2025 18:55, AISWARYA CHANDRAN via users wrote:
> Dear Quantum ESPRESSO developers and users,
>
> I am working on Ce₂O₃ and trying to reproduce the experimentally 
> reported band gap (~2.4–2.7 eV) using LDA+U in combination with 
> spin–orbit coupling (SOC) in Quantum ESPRESSO. However, despite using 
> U = 6 eV and including SOC via fully relativistic pseudopotentials, 
> the computed band gap is significantly underestimated (around 1.1 eV).
>
> Here are the key points of my setup:
>
> - Code version: QE 7.X
> - Pseudopotential: Ce.pz-spdfn-rrkjus_psl.1.0.0.UPF (fully relativistic)
> - Functional: PZ-LDA
> - Hubbard_U applied: U = 6.0 eV on Ce 4f
> - Calculation type: SCF → bands
>
> - Structure: 5-atom primitive cell from literature (a = 3.87 Å, c/a = 
> 1.522)
> - `nbnd = 41`
> - Gap is computed using `bands.x` and visual inspection
>
> Despite these, the gap remains ~1.1 eV. Literature reports higher 
> values for similar setups. I also observe that relaxing the cell via 
> `vc-relax` makes the lattice smaller, which further reduces the gap. 
> Using experimental lattice constants does help, but not enough to 
> reach the expected values.
>
> Could you please advise on:
> 1. Whether this pseudopotential is appropriate for SOC + LDA+U 
> calculations on Ce?
> 2. Any suggestions on correcting the band gap underestimation?
> 3. Whether a better suited pseudopotential or approach (e.g., using 
> GGA+U, HSE, etc.) is recommended? I am attaching the input file
> 4. I tried the same with Ce.pz-spdn-rrkjus_psl.1.0.0.UPF it gives band 
> gap of 3.7 eV. shall i proceed with spdn?
>
> I would greatly appreciate your insights.
>
>
> -- 
> *Thanks and with Regards, *
> *Aiswarya Chandran*
> *Research Scholar
> *
> *Laboratory for Advanced Materials and Applied Research
> *
> *Department of Chemistry*
> *NIT Calicut, Kerala, India, 673 601*
>
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-- 
Dr. Lorenzo Paulatto
IR IMPMC - CNRS UMR 7590 / Sorbonne Université / MNHN
phone: +33 (0)1 442 79822 / telegram: lpaulatto
http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
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