[CP2K-user] [CP2K:22331] Re: Stabilizing/finding C2v-like Be_Ga acceptor state in CP2K HSE calculations

[Johann Pototschnig]

There is the option to remain in the same space group:
KEEP_SPACE_GROUP
https://manual.cp2k.org/trunk/CP2K_INPUT/MOTION/GEO_OPT.html#CP2K_INPUT.MOTION.GEO_OPT.KEEP_SPACE_GROUP
but they require a C2v space group to start with. 

There are additional constrains possible, but for a 72 atom system, they 
are not useful:
https://manual.cp2k.org/cp2k-2024_2-branch/CP2K_INPUT/MOTION/CONSTRAINT.html




On Saturday, July 18, 2026 at 3:27:10 PM UTC+2 Asif Billah wrote:

> Hi CP2K community,
>
> I am studying Be substituting Ga in wurtzite GaN using CP2K/GPW with HSE + 
> ADMM. 
>
> Based on previous resources, it is possible to obtain three different 
> configurations for Be in GaN.
> C3v: one long axial Be-N bond
> pseudo-C3v: one long basal Be-N bond
> C2v: distortion of the two adjacent acceptor-nitrogen bonds, not a single 
> bond is significantly long
>
> I am trying to locate the shallow C2v like acceptor configuration, but so 
> far my relaxations either converge to the deep C3v or pseudo-C3v polaronic 
> states.
>
> I have already found stable/converged neutral C3v and pseudo-C3v branches. 
> In the 72-atom cell at α = 0.335, the pseudo-C3v branch is with one long 
> basal Be-N bond around 2.56 Å. The C3v branch has one long axial Be-N bond 
> around 2.59 Å. These two states can be obtained easily where the initial 
> structure will be by distorting the Be atom such that there will be a long 
> basal or axial bond. 
>
> The shallow C2v state of Be_Ga acceptor results from the distortion of the 
> two adjacent acceptor-nitrogen bonds, having the C2v symmetry of the atomic 
> structure. The hole in this case is anisotropically delocalized in the 
> (0001) wurtzite plane of GaN.
>
> Hence, for the C2v search, I am giving an initial structure by distorting 
> the Be atom along a bisector direction involving the axial N neighbor and 
> one basal N neighbor. However, it starts to collapse to the C3v/pseudo-C3v 
> state.
>
> My question is: does anyone have advice on how to better initialize or 
> constrain the search for the shallow C2v acceptor state in CP2K? Is it 
> possible to obtain C2v state? In particular, I would appreciate suggestions 
> on:
>
> 1. Better initial distortion pattern for C2v in wurtzite GaN
> 2. Better spin/localization initialization for the hole
> 3. Any CP2K-specific GEO_OPT/HSE/ADMM settings that help avoid collapse 
> into C3v or pseudo-C3v
>
> Any guidance would be very helpful. Thank you!
>
> Ps:
> My current setup is:
>
> System: Be_Ga in wurtzite GaN
> Cell sizes tested: 72 atoms, moving to 108 and 300 atoms
> Run type: GEO_OPT, fixed cell
> Functional: HSE-style screened hybrid
> alpha = 0.335
> omega = 0.1058354422 bohr^-1   # approximately 0.2 Å^-1
> MGRID cutoff = 600 Ry
> REL_CUTOFF = 60 Ry
> SCF: OT-CG, FULL_ALL preconditioner
> q = 0: UKS TRUE, MULTIPLICITY 2
> q = -1: UKS FALSE
> HFX: four-center HFX + ADMM
>
> Basis/pseudopotential setup:
>
> Ga: DZVP-MOLOPT-PBE0-GTH-q13 + admm-dz-q13, GTH-PBE0-q13
> N : DZVP-MOLOPT-PBE0-GTH-q5  + admm-dz-q5,  GTH-PBE0-q5
> Be: DZVP-MOLOPT-PBE0-GTH-q4  + admm-dz-q4,  GTH-PBE0-q4
>

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