You should figure out how the two materials match at the interface. This can be 
straightforward or quite hard, depending on the system.
If the metallic substrate surface is described by the same Bravais lattice as 
the semiconductor (e.g. you are interfacing two cubic or two
hexagonal materials), you easily choose the surface unit cell and must take 
care only of the lattice parameter. One choice is based on the fact that,
if one of the two materials plays the role of the substrate and a thin layer of 
the other material is deposited on top, you expect that the thin layer
will not significantly modify the thick substrate lattice parameter, and you 
use the substrate lattice parameters. If this is not the case, the lattice 
parameters
choice will depend on the specific system, and you could eventually choose to 
optimise them to get the lowest energy interface.

There are other interfaces where also the choice of the surface Bravais lattice 
is not straightforward because it requires to appropriately identify
surface supercells of both materials, in some cases after a rotation of one of 
the two.

You might find lot of examples in the literature, just to mention one I studied 
some year ago [J. Phys.: Condens. Matter 27 (2015) 015006], if you
interface the ZnO (0001) surface with the (111) surface of metals like Ag, Al, 
Pd, Pt, both have an hexagonal unit cell, so you can easily build for 
both the 1x1 surface hexagonal unit cell. However, it turns out that the two 
lattice parameters are quite different, thus resulting in a significant
mismatch at the interface, inducing strain that can modify the interface 
properties. A much lower interface mismatch is obtained by matching
the sqrt(3) x sqrt(3) R30 ZnO(0001) surface, with the 2x2 metal (111) surface. 
The former is obtained after a suitable rotation of the ZnO (see Fig. 2
of the mentioned paper).
This has nothing to do with what you want to study, but is to mention an 
example of what one should have in mind when setting up the surface
unit cell of an interface.

One you identify the geometry of your interface, than it is easy to set up the 
QE calculation, you choose the ibrav value that fits your needs (in the 
mentioned
example hexagonal lattice), with special care on the lattice parameter in the 
direction perpendicular to the surface, because a sufficiently large
vacuum space must separate two periodic replicas of the slab to avoid spurious 
interactions. If in the place of specifying the lattice using ibrav>0 and 
celldm(*)
you choose to use the CELL_PARAMETERS card, then you must just identify the 
three lattice vectors that describe the slab supercell, both in the directions
parallel and perpendicular to the surface (so they are still three 
three-dimensional vectors).

Concerning degauss, it is quite likely that if you start from a metal and a 
semiconductor, the metal will remain a metal after the interface is formed, so 
you
use degauss just in the same way as you do for calculation of the bulk metal.

More help could be provided if you give more details about the system you want 
to investigate.

Giovanni


> On 21 Sep 2016, at 18:52, Lorenzo Pedrazzetti <lorenzo.pedrazze...@polimi.it> 
> wrote:
> 
> Thanks everyone for the advices.
> But I think I am facing a bigger problem. I would like to analyse the 
> interface between a metal and a semiconductor, but many issues come to my 
> mind:
> 1. how should I define the system? I will have a system requiring smearing 
> while the other won't;
> 2.how to relate celldm() to the correct system?
> 3.still the building of "CELL_PARAMETERS" card is obsure to me for this kind 
> of mixed system.
> Thank anyone who wants to help.
> Lorenzo.
> 
> Sent from my ASUS
> 
> -------- Messaggio originale --------
> Da:Giuseppe Mattioli 
> Inviato:Wed, 21 Sep 2016 15:39:03 +0200
> A:pw_forum@pwscf.org <mailto:pw_forum@pwscf.org>
> Oggetto:Re: [Pw_forum] mixed supercells
> 
> 
> And if you have no experimental hint of the behavior of cell parameters at 
> the interface you can start by averaging the parameters, with a (hopefully 
> little) strain of both materials.
> 
> HTH
> 
> Giuseppe 
> 
> On Wednesday, September 21, 2016 03:09:49 PM Jess Wellendorff wrote:
> > The VNL "Interface Builder" would make it very easy for you to build a
> > minimum-strain interface:
> > 
> > http://docs.quantumwise.com/tutorials/ag_au_interface/ag_au_interface.html 
> > <http://docs.quantumwise.com/tutorials/ag_au_interface/ag_au_interface.html>
> > 
> > On 09/21/2016 03:04 PM, Lorenzo Paulatto wrote:
> > > Dear Lorenzo,
> > > you cannot specify different periodicity, you have to build a
> > > super-cell, as big as the 3x3 cell of the first material, or the 2x2
> > > cell of the second, then you fill it with 9 units of the first
> > > material and 4 of the other. This can be done by hand (the first time
> > > it'll take you an hour or two), or there are some softwares that can
> > > help you I have never used one, as hence I cannot recommend any.
> > > 
> > > 
> > > On 21 September 2016 at 14:46, Lorenzo Pedrazzetti
> > > <lorenzo.pedrazze...@polimi.it <mailto:lorenzo.pedrazze...@polimi.it> 
> > > <mailto:lorenzo.pedrazze...@polimi.it 
> > > <mailto:lorenzo.pedrazze...@polimi.it>>>
> > > 
> > > wrote:
> > >     Good afternoon,
> > >     
> > >     I am trying to model the interface between two solid with
> > >     different cella parameter. By looking at their experimental cell
> > >     value I am supposed to stack a 3x3 slab onto a 2x2 slab made of
> > >     different material.
> > >     
> > >     My problemi s the “CELL_PARAMETERS” card; i found some
> > >     difficulties to understand how to specify the different
> > >     periodicity of the two materials.
> > >     
> > >     Thanks a lot,
> > >     
> > >     Lorenzo.
> > >     
> > >     
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>    Giuseppe Mattioli                            
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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.cant...@spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
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