[Pw_forum] SURFACE RECONSTRUCTION USING RELAX CALCULATION

giuseppe.mattioli at mlib.ism.cnr.it giuseppe.mattioli at mlib.ism.cnr.it
Wed Mar 10 12:42:49 CET 2010


On the top of what Giovanni said (which is already very interesting) I  
would add that, in my experience, it is very, VERY difficult to study  
all but very simple surface reconstructions without a (minimal)  
experimental support. In other words, if you do not know, for example,  
whether the surface matches the periodicity of the bulk lattice by  
means of (at least) LEED patterns or (and this is better) LEED and STM  
patterns, you can try an 1x1 model and stop there... So, I suggest you  
to look for some experimental papers, before you start your  
theoretical investigation.

Yours

Giuseppe

ISM-CNR

Quoting Giovanni Cantele <giovanni.cantele at na.infn.it>:

>
> On Mar 10, 2010, at 10:35 AM, mohnish pandey wrote:
>
>> Dear users,
>>                    I want to get the results for surface   
>> reconstructions computationally, but the problem is as we have   
>> specify the "ibrav" i.e symmetry of the system the relax   
>> calculation only optimizes the structure within the given symmetry.  
>>  Can anybody suggest a way to model reconstructions .
>> Thanks in advance,
>> MOHNISH
>>
>> --
>> Mohnish Pandey
>> Y6927262,4th Year dual degree student,
>> Department of Chemical Engineering,
>> IIT KANPUR
>> 09235721300
>>
>
> Well, I think that a "very general" answer is difficult to give   
> (maybe someone more expert could try). What one usually does in   
> studying surfaces, is try to identify,
> step by step, possible reconstructions and look at the lower energy ones.
>
> In practice, for a given surface direction (e.g. the silicon 100   
> surface) you first identify the "minimal" unit cell (size and   
> symmetry) in the plane parallel to the surface. Then, you must decide
> the number of atomic planes that you want to include in your   
> calculation (this will fix the lattice constant in the direction   
> orthogonal to the surface). This
> will represent a so-called 1x1 model, meaning that your "surface   
> unit cell" is made by just one unit cell. Possible reconstruction   
> (which means atomic rearrangement of the atoms with respect to the   
> ideal positions) will be, of course, the same in all surface unit   
> cells.
>
> More complicated reconstructions might arise because:
>
> i) there can be a surface atomic displacement different in neighbor   
> unit cells (for example, in the case of the cited Si surface two Si   
> atoms in different 1x1 cells approach to each other to form a dimer,  
>  this gives you a 2x1 reconstruction; then two neighbor dimers can   
> show different tilt with respect to the surface plane, this gives a   
> 2x2 reconstruction, etc.); to include
> this effect in your calculation you replicate your surface unit cell  
>  (building a "supercell") in one or both directions parallel to the   
> surface, so as to build 1x2, 2x1, 2x2, 1x4, etc. models. Of course   
> one should
> be driven by either already known results, experimental evidences,   
> etc. or by physically/chemically meaningful guesses. In this case   
> one should also be careful to break possible symmetries that would   
> prevent from reaching the energy minimum. Again, in the case of the   
> Si(100) surface, if you just replicate the 1x1 unit cell along one   
> surface direction, to build the 2x1 model, the code will just give you
> a structure identical to the 1x1 model, with an energy 2 times   
> larger; so, what you do is to move (even by a small amount) the   
> atoms you expect will move, namely, the two Si atoms which dimerize.  
>  Alternatively, you can randomize the position of some/all atoms,   
> just to break the symmetry.
>
> ii) there can be a reconstruction where there are more/less atoms   
> than you would expect in the "ideal" surface, a typical example is   
> the case of O vacancies in oxide surfaces.
>
> Because the plane-wave calculation, as you probably already know, is  
>  by construction periodic in all directions, you must be sure to   
> include, along the direction orthogonal to the surface a
> "vacuum space" (that means to increase the corresponding lattice   
> parameter) to prevent two consecutive slabs from interacting.   
> Convergence must be checked against the vacuum space.
> Also, if you want to retrieve the "bulk surface" properties, the   
> thickness of your slab should be larger enough (this is controlled   
> by the number of atomic planes included in the unit cell) to prevent
> opposite sides of a given slab from interacting with each other.
>
> Another issue is that the "optimal" surface unit cell might show a   
> completely different "symmetry" than the bulk one. For example, in   
> the case of Au/Pt/etc. (111) surfaces, the bulk crystal is a cubic
> fcc lattice, whereas the minimal surface unit cell can be   
> represented using an hexagonal lattice.
>
> Hope this helps,
>
>     Giovanni
>
>
> --
>
> Giovanni Cantele, PhD
> CNR-SPIN and Dipartimento di Scienze Fisiche
> Universita' di Napoli "Federico II"
> Complesso Universitario M. S. Angelo - Ed. 6
> Via Cintia, I-80126, Napoli, Italy
> Phone: +39 081 676910 - Fax:   +39 081 676346
> Skype contact: giocan74
>
> ResearcherID: http://www.researcherid.com/rid/A-1951-2009
> Web page: http://people.na.infn.it/~cantele
>                      http://www.nanomat.unina.it
>
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