Re: [SIESTA-L] Optical.Mesh

[A.S.]

Thanks Martin, I  have read Siesta codes myself and I am aware of the two grids and the necessity for the grid used for the interpolation of wavefunction to be overly dense so that the transition amplitudes computed on the optical mesh have correct values - just wanted to make sure that what I am telling my colleagues about convergence is true. Andrey -----Original Message----- From: Martin Blaber <[EMAIL PROTECTED]> Sent: Jul 22, 2007 8:38 PM To: SIESTA-L@listserv.uam.es Subject: Re: [SIESTA-L] Optical.Mesh Hi Andrey, I believe you are correct. However, now the 'steady state solution' that you come to is limited by the number of k points specified in the Monkhorst Pack grid. i.e. If your optical mesh is infinitely dense, the Monkhorst Pack grid limits the accuracy of the integral over the BZ. In my experience the variations are due to insufficient mesh sampling up to a large optical mesh, say 45^3, and then the solution starts to oscillate and doesn't improve with increased sampling. However, I use 0.1eV broadening, which may in itself cause the oscillation and is much higher detail than one could hope to see in the experimental permittivity of a bulk material. I shall have to go and read the code and get back to you for more info. Cheers, Marty A.S. wrote: Hi Martin, Thanks for this information. The optical mesh in Siesta is then simply a set of k-points at which the intraband transitions for evaluating eps(w) are calculated, and its refinement allows approximating the volume integral over the Brillouin zone with a higher accuracy - am I correct? Suppose, I calculate the properties of a solid using a 5x5x5 Optical.Mesh, then 10x10x10, then ... 60x60x100, and first I find big variations in eps(w) for between 5^3 and 10^3. Then, as Optical.Mesh becomes denser, I find that eps(w) have converged to some "steady state solution". Does this mean that the variations in eps(w) at low k-point density are due to the insufficient sampling rate and as the sampling rate increases, I have a more accurate value of the integral over the BZ? Andrey -----Original Message----- From: Martin Blaber <[EMAIL PROTECTED]> Sent: Jul 19, 2007 6:47 PM To: SIESTA-L@listserv.uam.es Subject: Re: [SIESTA-L] Optical.Mesh Hi Andrey, For bulk materials a Monkhorst Pack grid of 21^3 is sufficient to stop the eps(w) for a constant optical mesh (Depending on the system). Whereas an optical mesh grid of 41^3 vs 45^3 still shifts eps(w) around a bit for constant Monkhorst Pack grid. The broadening also affects what can be considered converged. I generally keep the optical broadening very low as you can always broaden more but never less. However, this does mean an optical mesh of 45 isn't sufficient for convergence. Hope this helped. Cheers, Marty --------------------- Martin Blaber Institute for Nanoscale Technology Dept. of Physics and Advanced Materials University of Technology, Sydney [EMAIL PROTECTED] wrote: Hi all, I am trying to find some description of the Optical.Mesh variable which is more detailed than the one given in the Siesta manual. Some of my colleagues insist that the convergence of optical properties eps(w) with respect to the number of k-points in the integral wrt k over the BZ has to be shown not based on the vaues in the Optical.Mesh but rather based on the values of MonkhorstPack_grid. How can I prove the opposite, i.e. that BZ sampling grid refinement for optical property calculations must be based on the Optical.Mesh variable? Thanks, Andrey Semichaevsky, CEMOS, ECE Dept., UMass Lowell ________________________________________ PeoplePC Online A better way to Internet http://www.peoplepc.com ________________________________________ PeoplePC Online A better way to Internet http://www.peoplepc.com