[QE-users] Relaxation in silicon with a divacancy

[Paolo Giannozzi]

Are you sure that PBE (or any GGA-like functional) works well for your 
case? Is this a case in which a hole localises on a bond? GGA may tend 
to favor configurations in which the hole is uniformly spread, due to 
self-interaction errors. I encountered a similar case many years ago (Al 
defect in SiO2), wrote a paper with wrong results :-( and have been more 
careful with GGA since

Paolo

On 20/08/2024 09:59, David Fernandes Machado wrote:
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> Hello everyone,
> 
> I am currently studying a supercell composed of 510 silicon atoms, 
> including a divacancy. I am conducting a relaxation calculation using 
> the PBE pseudopotential. Since silicon is tetravalent, each divacancy 
> causes three neighboring silicon atoms to lose a bond. At a total charge 
> of 0, I anticipate two distinct equilibrium structures.
> 
>  1. The three silicon atoms form two bonds, resulting in two short
>     distances (~3 Å) and one longer distance (~3.5 Å) between them.
>  2. The three silicon atoms form only one bond, leading to one short
>     distance and two longer distances between them.
> 
> To achieve these two structures, I initialize the relaxation calculation 
> by adjusting the distances between the three atoms to resemble the 
> expected configurations. To expedite the calculation, I perform the 
> relaxation in two stages: the first with force convergence (forc_conv) 
> set to 10−310^{-3}10−3 and a wavefunction cutoff energy (ecutwfc) of 10 
> Ry, and the second with forc_conv set to 10−410^{-4}10−4 and ecutwfc of 
> 25 Ry.
> 
> However, despite trying various approaches, I consistently observe three 
> long distances of approximately 3.4 Å at the end of the second 
> relaxation step, even though the structure appears correct after the 
> first step.
> 
> Has anyone encountered a similar issue? Any suggestions on how to 
> resolve this problem would be greatly appreciated.
> 
> Thank you.
> 
> Best regards,
> 
> 
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-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216
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