[CP2K-user] [CP2K:22067] Energy Difference Between OT and Diagonalization in CP2K

[yis...@163.com]

Thank your very much

在2026年1月26日星期一 UTC+8 17:21:55<Jürg Hutter> 写道：

> Hi
>
> Please note that you forgot to specify the vdW correction (D3) in the 
> k-point
> calculations.
> Make sure that the vdW energy is also converged in both calculations 
> (cutoff!).
>
> best
> JH
>
> ________________________________________
> From: cp... at googlegroups.com <cp... at googlegroups.com> on behalf of 
> yis... at 163.com <yis... at 163.com>
> Sent: Monday, January 26, 2026 4:23 AM
> To: cp2k
> Subject: [CP2K:22065] Energy Difference Between OT and Diagonalization in 
> CP2K
>
> To the CP2K Development Team,
>
> Hello!
>
> I would like to raise another question: why is there a discrepancy in the 
> calculated energies when using the OT (Orbital Transformation) method 
> compared to the diagonalization method?
> I started with the NiO₂ unit cell (5.6512 Å × 5.6512 Å × 4.3886 Å). Due to 
> its small size, I expanded it into supercells and performed calculations 
> using the OT method. As shown in the attached table, I constructed 
> supercells of sizes 2×2×2, 3×3×3, 3×3×4, and 4×4×4. The energies per NiO₂ 
> unit cell obtained were –805.2913786 Ha, –805.2931813 Ha, –805.2932437 Ha, 
> and –805.2931291 Ha, respectively, with relative energy differences of 
> –1.803×10⁻³ Ha, –6.232×10⁻⁵ Ha, and 1.145×10⁻⁴ Ha. The results indicate 
> that the energy converges to –805.2932437 Ha per unit cell when the 
> supercell is expanded to 3×3×4.
>
> [微信图片_20260126112239_276_77.png]
> Additionally, I tested the diagonalization method with k-point sampling. 
> Using k-point grids of 4×4×4, 5×5×5, 6×6×6, and 7×7×7, the energies per 
> NiO₂ unit cell were –805.2394556 Ha, –805.2394471 Ha, –805.2395249 Ha, and 
> –805.2394426 Ha, respectively, with relative differences of 8.512×10⁻⁶ Ha, 
> –7.782×10⁻⁵ Ha, and 8.234×10⁻⁵ Ha. The energy converges to –805.2394556 Ha 
> per unit cell with a 4×4×4 k-point grid.
> Although both methods appear to converge individually, there is a 
> significant discrepancy of about 0.0538 Ha (~1.46 eV) between the two 
> results, which cannot be overlooked. Could you please help explain the 
> possible reasons for this difference?
> For reference, I have attached the relevant test results and input/output 
> files:
> cp2k_K444.inp and cp2k_K444.out: input and output files for the 
> diagonalization calculation with a 5×5×5 k-point grid.
> cp2k_SP334.inp and cp2k_SP334.out: input and output files for the OT 
> method calculation with a 3×3×4 supercell.
> nio2.cif: the unit cell file used in the calculations.
>
> A figure summarizing the energy convergence with respect to supercell size 
> and k-point sampling.
>
> I welcome any feedback or suggestions from the developers. Thank you for 
> your time and assistance!
>
> Best regards,
> yisichi
>
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