Of course, in principle slabs should converge to bulk epsilon. But:
In your slabs with a k-mesh of 69x69x1 you are using "effectively" a k-mesh of 69x69x39 instead of a 69x69x69 mesh. In addition, in the z direction you use "root-sampling" instead of tetrahedra method. It is like integration with the rectangular-rule instead of a trapezoidal rule. Try fcc-Al with a small tetragonal distortion during setup, so that you get only 16 sym.ops. then change c/a back to 1 and use a kmesh of 69x69x39 and compare the dielectric function to the 2 times 69-mesh. (This mimics the k-mesh problem, but still there s the integration method !!). You probably need even more layers .... Am 26.11.2013 04:51, schrieb phlhj phlhj:
Dear Prof. Blaha, Thanks so much for your suggestion. I tried bulk Al supercell with 39ML without vacuum with the same k-mesh as used in 39 ML thin film supercell. In fact I get the same results for plasma frequency and dielectric function as those from Al unit cell with only one Al atom. I think the k-mesh of 61x61x1 I used in my calculation is dense enough to give a precise result. The main difference for the dielectric function between thin film geometry and bulk geometry is at the low energy range (<1.2eV). I researched some paper for studying the anisotropic surface reflectance in semiconductor surface, say, GaAs(110). Even 15 atomic layers are used in the LDA calculation but still some difference around the band gap regime for the dielectric function is found between surface calculation and bulk calculation. I think the difference I encountered for teh dielectric function between slab Al(111) and bulk Al might be similar to the case in semiconductor system. However, from the physical point of view, it's hard to understand why there is still appreciable difference out there even though very thick film is used. Physically the dielectric function of the very thick slab should converge to that in the bulk counterpart. Thank you so much for sharing any understanding about this, Wenmei 2013/11/24 Peter Blaha <[email protected] <mailto:[email protected]>> As you probably know, the dielectric function of Al converges VERY slowly with respect to the k-mesh. When you do slab calculations, you include the surface effect, but you also replace the periodicity in k-z (and thus the k-mesh in k-z) to a backfoldung according to your slab. Even a 39 ML slab corresponds probably not to a very large k-z mesh and in addition the integration over k-z is limited to a "root"-sampling instead of the tetrahedron method. I could even imagine large numerical problems in this 2-D integration using a 3-D algorithm in joint due to large degeneracy of the tetrahedra. At least you could differentiate between "integration problems" and surface effects by using a 39-layer bulk structure (i.e. remove the vacuum in your supercell, so that you get 3D-Al again, but restrict yourself to 1-k point in k-z) and compare the resulting eps to bulk Al (with 1 atom/cell and good k-meshes. Am 23.11.2013 16:54, schrieb phlhj phlhj: Dear all, I was trying to calculate the optical properties of Al(111) slab. For the bulk FCC Al, I can reproduce the dielectric functions and plasma frequency very precisely reported in literature before. However, I did find some difference between the slab dielectric functions and the corresponding bulk values. Especially even though I used very thick slab, say 39MLs, in the low photo energy range (<1eV), the imaginary part is much larger than the bulk. I doubt this may be related to the band-folding and symmetry reduction in the direction normal to the surface. Also, I found the plasma frequency of the slab is smaller than the bulk plasma frequency. Mathematically, this behavior of the imaginary parts of the interband and intraband transitions contributions seems to be able to be understood from the f-sum rule. 1) However, physically it's hard to believe, because when the slab thickness is very thick for example the 39MLs used in my test calculation, the slab wavefunctions should be very very close to the bulk wavefunction except in the very thin slab/vacuum interface region. This should give us the dielectric functions for the slab which are very very close to the bulk values. This argument should be also true for the slab plasma frequency. 2) If the different values are because of the surface slab structure we used in the calculation, which indeed breaks the translational symmetry in the normal direction. Then the question is that in real experiment because the sample always is finite with the boundary surface, how can we get the dielectric information really for the ideal bulk rather than the slab similar as that mentioned above. Or in calculating dielectric function, when should we use bulk geometry? when should we use slab geometry? Thanks a lot for any idea. Wenmei _________________________________________________ Wien mailing list [email protected].__at <mailto:[email protected]> http://zeus.theochem.tuwien.__ac.at/mailman/listinfo/wien <http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien> SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected].__at/index.html <http://www.mail-archive.com/[email protected]/index.html> -- ------------------------------__----------- Peter Blaha Inst. Materials Chemistry, TU Vienna Getreidemarkt 9, A-1060 Vienna, Austria Tel: +43-1-5880115671 <tel:%2B43-1-5880115671> Fax: +43-1-5880115698 <tel:%2B43-1-5880115698> email: [email protected] <mailto:[email protected]> ------------------------------__----------- _________________________________________________ Wien mailing list [email protected].__at <mailto:[email protected]> http://zeus.theochem.tuwien.__ac.at/mailman/listinfo/wien <http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien> SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected].__at/index.html <http://www.mail-archive.com/[email protected]/index.html> _______________________________________________ Wien mailing list [email protected] http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected]/index.html
-- ----------------------------------------- Peter Blaha Inst. Materials Chemistry, TU Vienna Getreidemarkt 9, A-1060 Vienna, Austria Tel: +43-1-5880115671 Fax: +43-1-5880115698 email: [email protected] ----------------------------------------- _______________________________________________ Wien mailing list [email protected] http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien SEARCH the MAILING-LIST at: http://www.mail-archive.com/[email protected]/index.html
