Hi Giovanni, Thank you for spotting this error. We?ll change the input, use cdlldm(1) and try again.
Jack School of Chemistry University of Birmingham On 15 Nov 2013, at 14:46, Giovanni Cantele <giovanni.cantele at spin.cnr.it<mailto:giovanni.cantele at spin.cnr.it>> wrote: On Nov 15, 2013, at 2:53 PM, Jack Davis <JXD231 at bham.ac.uk<mailto: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 <Au_FCC.in>_______________________________________________ Pw_forum mailing list Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> http://pwscf.org/mailman/listinfo/pw_forum 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<mailto: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<http://www.nanomat.unina.it/> _______________________________________________ Pw_forum mailing list Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org> http://pwscf.org/mailman/listinfo/pw_forum
