Hi Emine, Thank you for clearing up what I should be converging. I wasn't too sure if I should be trying to converge the smearing and k-point combinations to a single value. I'll give your technique a go, and also incorporate the forces as mentioned by Nicola. I'm just on my university library and should have Stefano's paper soon.
All the best, Ben > Hi Ben, > > the idea is to calculate the "converged energy" wrt "smearing width." > So, the procedure as I understand is > 1.choose a smearing width > 2.converge the energy at that smearing width with by choosing an > appropriate k mesh > 3.repeat it for other values of smearing width. > > You need to be quite generous with the range of smearing width you > explore. > Your values span a very narrow range where smearing is __very__ small. > Although it depends on the material, I find a range of 0.1 to 0.005 is > generally a good starting point. > Guess I would do calculations at around 0.1, 0.05, 0.03, 0.02, 0.01, > 0.005 if i was exploring the correct behaviour here.. > > An example: > To understand if you have spanned a good range for the k point grids > you can afford, > look for "merging lines": > As you increase the smearing, the convergence is gonna get to be more > easily reached with even smaller k point meshes. so you would be able > to see > * lines for 18, 16 to merge to same value lets say -154.062, at width > 0.005 ; > * then as you increase smearing width further to 0.02 you would see 18 > 16 14 12 mesh lines will all merge to same value of -154.061, > * and continuing, at width 0.1 all lines 18 16 14 12 8 4 will converge > to energy value -154.058 > > that is what we mean by "converged energy" at each width: > when k point lines merge it means that you have reached convergence > wrt k mesh for that width. > > in the above made-up example, > smearing width versus converged energy would look like: > > 0.005 -154.062 (18,16 merged) > 0.01 -154.062 (18 16 ) > 0.02 -154.062 (18 16 14) > 0.03 -154.061 (18 16 14 12) > 0.05 -154.060 (18 16 14 12 8) > 0.1 -154.058 (18 16 14 12 8 4) > > so 0.02 width and 14 kmesh could be considered converged with the > given precision here. > > if you have chosen a very small width range (for the k grids you can > afford), you will see separate, flat lines as you do in your figure > instead. Anyways, next time such separate flat lines can be your cue > to explore wider ranges of smearing width. > > Note that in the above example, i have always reduced the energy as i > reduced the smearing width. this does not have to be so. Indeed in > Nicola's thesis and Stefano's paper on phonons w metallic systems, you > can see how different ways of smearing would depend differently on the > shape of the density of states at the fermi level to determine this > behaviour. > > Also note that in the above example i have used energy as a > convergence criterion but you could have as well used forces as Nicola > suggested. > > You already have Nicola's thesis link, and here is Stefano's paper: > http://prb.aps.org/abstract/PRB/v51/i10/p6773_1 > > best > emine kucukbenli, postdoc at theos, epfl, switzerland > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20130228/49738d39/attachment.html
