[Pw_forum] Reconstruction of Si 001 surface

[Giovanni Cantele]

Dear Bipul,

you are not actually doing a vc-relax calculation, because in your input calculation=relax. However, this is exactly what I would do, at least to start (for example, for a vc-relax calculation I guess that pw.x will try also to optimise the cell size along the z direction, where the dimension must be fixed according to the amount of vacuum you want to use).

In order to get the 2x1 reconstruction you should slightly move (with respect to their bulk positions) the two atoms expected to form a dimer along the proper direction. Otherwise, pw.x will find more symmetry than that of the 2x1 reconstruction of the Si 001 surface, and will keep that symmetry during the whole calculation. Consider that, if you try to visualise the structure using XCrysDen, it seems that you are using a supercell larger than the minimal one needed to simulate that surface (I see two “dimers” within your supercell, so with that supercell you can also simulate the 2x2 reconstruction). The easiest way to get the minimal cell is to rotate, as far as I remember, the atoms by 45 degrees in the x-y plane (around the z axis), so that the dimer gets parallel to, let’s say, the y direction. In this way it is also sufficient to replicate that supercell along x and y to get also 2x2 or 4x2 reconstructions.

Finally, you should check whether using an asymmetric slab model (because you fix the bottom layers of the slab) induces artificial dipole that must be corrected using dipfield and related variables in the input files.

Hope this helps.

    Giovanni


> On 22 Sep 2015, at 12:12, Bipul Rakshit <bipulrr at gmail.com> wrote:
> 
> Dear Pwscf users,
> I want to study the Si 001 surface reconstruction. I am sending the input of 2x1 Si (001) surface. I used 16 Ang Vaccuum between the slabs along z-direction. I am using calculation = 'relax'. In the literature i find that 2x1 reconstruction for Si (001) surface is there. But with the supplied input after running the calculation, I am not getting the dimers which is expected to form in 2x1 reconstruction.
> 
> I do use calculation=vc-relax, but the calculation is not converging after 100 iterations and get killed.
> So what changes I have to made in the input so that i can get the 2x1 dimer formation in the reconstructed surface.
> 
> kindly help me in this regard.
> 
> -- 
> Dr. Bipul Rakshit
> Research Associate,
> Institute of Physics (IOP),
> Bhubaneswar- 751 005
> Orissa 
> India
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-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele