[Pw_forum] SURFACE RECONSTRUCTION USING RELAX CALCULATION

[mohnish pandey]

Dear Giovanni!
                       Thank you so much for you reply. As you suggested for
experimental evidence of the reconstruction I have got the reference below
which is experimental. This paper involves the structural change in gold
nanofilm for certain thickness in which (001) surface changes to (111)
surface.
                I  am unable to attach the paper because its size is large.
I hope your suggestion will definitely help.
Thank you very much to all of you
Sincerely,
MOHNISH
 " Phys. Rev. Lett. 82, 751–754 , Thickness Induced Structural Phase
Transition of Gold Nanofilm"


On Wed, Mar 10, 2010 at 3:40 PM, Giovanni Cantele <
giovanni.cantele at na.infn.it> wrote:

>
> 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://people.na.infn.it/%7Ecantele>
>                     http://www.nanomat.unina.it
>
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-- 
Mohnish Pandey
Y6927262,4th Year dual degree student,
Department of Chemical Engineering,
IIT KANPUR
09235721300
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