Lijun Zhang wrote: > 1.The highest mode should be about 3078 cm-1 (Th.Frauenheim Thin Solid > Films 272 (1996) 314-330), while I don't know what makes the optical > mode frequences so high, is it because of the thin vacuum layer?
The most likely reason for a 800 cm-1 discrepancy is that the system you study is different from the one that was experimentally characterized (provided your calculations are correct, of course). Maybe the expt one has a different reconstruction, no hydrogen, etc... Very difficult to say. > 2.How to determine the thickness of the slab and the thickness of the > vacuum? You increase them, separately, until the quantities you are interested in do not change anymore. Certain quantities (e.g. the highest optical frequency) converge fast - certain others (e.g. the zero-point motion contribution, or the vibrational free energy, integrated on all the phonons) are much slower to converge. Making sure that you converge with respect to wfc cutoff, charge density cutoff, k-point sampling (and smearing, in a metal), vacuum, slab thickness, self-consistency threshold, and phonon self-consistency threshold (these last two are *very* critical) is painful, slow, and *necessary*. Note that the acoustic frequencies at Gamma are the most difficult to converge (if I remember well our 2005 PRB Mounet and Marzari, to get them to 0 +/-1 cm-1 in bulk diamond we needed to use a cutoff for ultrasoft C of 100 Ry (!), and a dual of 28, i.e. a charge density cutoff of 2800 Ry). In practice, you never use such a high cutoff (have a look at out paper for the right numbers), but you enforce the acoustic sum rules (i.e. renormalize the interatomic force constants so that if you rigidly translate all atoms, the energy does not change). After imposing this physical symmetry, you'll find that the convergence with respect to cutoff is much better behaved. Of course, sometimes you might need accurate numbers for low frequency mode that are close to zero but not zero (e.g. some torsional modes in carbon nanotubes). Then, you just need to painstackingly verify convergence. > 3.I relaxed all the atom in the slab, should I fixed the middle 4 layers > (8 atoms)? Depends, again, on what you want to calculate. It might be better to not fix anything, fix a couple of layers in the middle, or a couple of layers in the bottom. I usually do not fix anything, so I have less parameters to worry about (which atoms to fix). Best luck, nicola PS: of course, have a look at the literature to get some sense of what is needed - the various phonon calculations in slabs by Claudia Bungaro, for metals, or by Pavone, for semiconductors. --------------------------------------------------------------------- Prof Nicola Marzari Department of Materials Science and Engineering 13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu
