[Pw_forum] Au (111) Surface Energy

[Giovanni Cantele]

On Nov 15, 2013, at 2:53 PM, Jack Davis <JXD231 at bham.ac.uk> wrote:

> Dear Forum,
> 
> I’m trying to calculate surface energies using PWscf, however, I am unsure of the methodology. To begin with we calculate the bulk energy of the FCC unit cell, using the attached input. We then calculate the energy of a 111 slab, 10 layers thick with a 6 atom vacuum gap. The surface energy is then calculated as,
> 
> Surface energy = Energy of slab - (bulk energy * 10) / 2. 
> 
> I think I am probably misunderstanding the energies PWscf is giving me. We’re getting some strange answers! 
> 
> Thank you,
> Jack Davis
> 
> School of Chemistry 
> University of Birmingham 
> 
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1) a=7.710 : are you sure that the Au ccc lattice constant is about 7-8 Angstrom?
Maybe I'm wrong, but as far as I remember it should be about 4 A. Maybe you meant
to give the lattice length in atomic units, but at this purpose use celldm(1) instead
(see PW/Doc/INPUT_PW.txt for more explanations)

2)  Energy of slab - (bulk energy * 10) / 2 : it should be  [Energy of slab - (bulk energy * 10)] / 2

3) the slab energy from the previous equation comes from the difference between two energies.
As such, if the two energies are not converged at the same level, you can have a systematic error which
can even significantly affect your results. In particular, you should check that both the slab and the bulk calculation
are EITHER fully converged with respect the k-point grid (within an error that should be much less that the surface
energy you expect) OR converged at the same level, meaning that you use two different k-point grids scaled in such
a way that the two calculations are affected by the same "error" (see for example PHYSICAL REVIEW B 80, 235407 􏰀2009􏰁
and references therein for different recipes to calculate the surface energy).

Giovanni

--

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Scienze Fisiche
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
                     http://www.nanomat.unina.it

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