Hello, Regarding the scissor for 1D and 2D structures, we have many reports in experiment. Especially for 1D structures since we have different diameters of for example wires with different bandgaps, can we consider scissor as follows: "optical scissor = experimental bandgap (for bulk) - calculated bandgap (for wire)" ?
I would be grateful to know this as well. Regards, Julie Smart Caltech University www.caltech.edu ________________________________ From: Marty Blaber <[EMAIL PROTECTED]> To: [email protected] Sent: Monday, December 8, 2008 9:59:41 AM Subject: Re: [SIESTA-L] correct optical calculation Hi Julie, The reason for the wiggles is generally because the integration mesh isn't fine enough for the broadening you have used. Lower broadening requires finer mesh. Having said that, if the experimental data looks similar with the calculation at 0.2 eV, you shouldn't have to go much below that. (Maybe try 0.1 eV and an optical mesh of 50? just to check) No need to increase the 1, should be fine as you have no periodicity in this direction. Your definition of scissor operator is correct! (Sorry, I should have been more concise) Cheers, Marty 2008/12/8 Julie Smart <[EMAIL PROTECTED]> Dear Marty, Happy to receive your guide. I have put broadening of 0.2 eV, since when I consider smaller amounts, the imaginary part of dielectric function goes to have alot of ups and downs which changes the results I expect. Should I again try to cgange it to smaller amounts? Also, for 2D structure I have: %block kgrid_Monkhorst_Pack 8 0 0 0.00 0 8 0 0.00 0 0 1 0.00 %endblock kgrid_Monkhorst_Pack So, I have chosen the mesh as: %block Optical.Mesh 40 40 1 %endblock Optical.Mesh should I still change "1" to higher amounts? Also, for the Scissor, as far as I got you we will have "optical scissor = experimental bandgap - calculated bandgap" ? Wait for your kind advice. Best wishes Julie Smart Caltech University www.caltech.edu ________________________________ From: Marty Blaber <[EMAIL PROTECTED]> To: [email protected] Sent: Monday, December 8, 2008 4:07:18 AM Subject: Re: [SIESTA-L] correct optical calculation Hi Julie, The scissor operator moves all the eigenvalues above the fermi energy (chemical potential) up by the value you specify, this is to compensate for the underestimation of the band gap. Usually you just choose it so that you're band gap matches experiment. The Broadening parameter specifies the size of the gaussian convolved with the transitions, and as such also defines the number of points that appear in the .EPSIMG file. As a rule of thumb you should try to keep the broadening reasonably low. You can always broaden the data further, but you can't deconvolve the gaussian out of the data (because of the correlation between gaussian width and number of points). I generally use a broadening of 0.02 eV. Don't forget that if you're dealing with metals, the plasma frequency converges slower than the transitions, so you should use large optical mesh. Your optical.Mesh should be the same style as you're monkhorst pack grid, but much higher density. To get accurate optical calculations, on gold for example, you should use a monkhorst pack grid of about 21, but you may have to go as high as 29 if you're unit cell is small. %block kgrid_Monkhorst_Pack 21 0 0 0.00 0 21 0 0.00 0 0 21 0.00 %endblock kgrid_Monkhorst_Pack And then use a mesh like this: %block Optical.Mesh 65 65 65 %endblock Optical.Mesh Note that you should test the convergence of these parameters. Also, the finer that your broadening is, the more points you need in optical kspace to get smooth data. The real part is more difficult, if you're using metals, you will have to guess the relaxation time. And you must check the convergence of the plasma frequency with increasing optical mesh. Cheers, MArty 2008/12/7 Julie Smart <[EMAIL PROTECTED]> Hi all, I have read all mailing archive of Siesta and also all the tutorials and the power points about optical properties. Still I have problem with Optical.Broaden and Optical.Scissor, seems that I have no good idea what they do although forexample I know what gaussian broadening is. To cut it short, I want to know usually what ranges of these two parameters ore suitable for bulk, 1D and 2D structures. Also, when I can get the best shape of imaginary and real part of dielectric function. It would be an honor to receive your advice. Best wishes Julie Smart Caltech University www.caltech.edu
