For almost 250 atoms per unit cell, a 1x2x2 k-points grid and assuming that you use a base of one atomic orbital per atom, you have 1000 eigenvalues. Obviously, this number will increase up to 4000 eigenvalues, if you have 4 atomic orbitals per atom. Under these conditions and assuming that the k-points grid is inverse proportional to the dimensions of your unit cell dimensions, then the convergence criterion should work fine. I believe that a 1x1x1 k-points grid might be also fine. You can check that easily and you can see the differences between a 1x1x1 and a 1x2x2 k-points grid. I believe that you will not find any significant differences.
Zacharias Fthenakis > Dear vZacharias, > > is your criterion also correct regarding convergence with resperct to the > k-grid? > > I'm asking because in this case I reach convergence already with a 1x2x2 > mesh and this seems to me a very little amount of grid-points. > > > > Thanks and Regards > > > > Stephan > > > > -----Original message----- > From: Ludwig, Stephan <[email protected]> > Sent: Wednesday 12th August 2015 20:00 > To: [email protected] > Subject: RE: [SIESTA-L] When is convergence reached? > > > > Thank you Zacharias > > > > -----Original message----- > From: [email protected] <[email protected]> > Sent: Wednesday 12th August 2015 19:45 > To: [email protected] > Subject: RE: [SIESTA-L] When is convergence reached? > > > Try to find the convergence in terms of energy per atom. What you will > find is that your energy is converged with accuracy less than 1meV/atom. > This accuracy is good enough for band structure calculations. Therefore, > convergence has already been reached for a 200 mesh cutoff. > > Zacharias Fthenakis > > >> Hi, >> >> thank you for the reply my unit cell is rather big (at least for a >> solid). >> Its an organic salt with 236 atoms. >> >> Cell volume = 2905.615438 Ang**3 >> >> : Cell vector modules (Ang) : 32.783000 11.179003 7.999052 >> >> >> >> In the first step I want to study bandstructure DOS and PDOS. >> >> Later optical excitations. >> >> >> >> Can you tell when I can call the system convergent wrt the total >> energy. >> >> >> >> Thanks and Regards >> >> Stephan >> >> >> >> -----Original message----- >> From: [email protected] <[email protected]> >> Sent: Tuesday 11th August 2015 18:52 >> To: [email protected] >> Subject: Re: [SIESTA-L] When is convergence reached? >> >> >> Hi Stephan, >> it depends on how big is your unit cell and what you want to study. >> >>> Hi, >>> >>> I've a question concerning the convergence criterion. >>> >>> Let's say I want to examine convergence of the total energy a system >>> with >>> respect to the meshcutoff. >>> >>> When is convergence reached? Is it reached when there is no change in >>> the >>> first digit after the comma >>> >>> like in the following example: >>> >>> meshcutoff total energy >>> >>> 150 -42994.1019 >>> >>> 170 -42994.116 >>> >>> 190 -42994.127 >>> >>> 210 -42994.133 >>> >>> >>> >>> or is it usefull to continue the test till there is no change in the >>> second digit after the comma? >>> >>> What is a sensible criterion for convergence? >>> >>> >>> >>> Thanks and regards >>> >>> >>> >>> Stephan Ludwig >>> >>> >> >> >> ********************************************************* >> Dr Zacharias G. Fthenakis >> Research Associate >> Institute of Electronic Structure and Laser (I.E.S.L.) >> Foundation for Research and Technology Hellas (FO.R.T.H.) >> Vassilika Vouton P.O. Box 1527 71003 Heraklion Crete Greece >> Phone +30 2810 391824 >> FAX +30 2810 391305 >> webpage: http://esperia.iesl.forth.gr/˜fthenak >> ********************************************************** >> >> >> > > > ********************************************************* > Dr Zacharias G. Fthenakis > Research Associate > Institute of Electronic Structure and Laser (I.E.S.L.) > Foundation for Research and Technology Hellas (FO.R.T.H.) > Vassilika Vouton P.O. Box 1527 71003 Heraklion Crete Greece > Phone +30 2810 391824 > FAX +30 2810 391305 > webpage: http://esperia.iesl.forth.gr/˜fthenak > ********************************************************** > > > ********************************************************* Dr Zacharias G. Fthenakis Research Associate Institute of Electronic Structure and Laser (I.E.S.L.) Foundation for Research and Technology Hellas (FO.R.T.H.) Vassilika Vouton P.O. Box 1527 71003 Heraklion Crete Greece Phone +30 2810 391824 FAX +30 2810 391305 webpage: http://esperia.iesl.forth.gr/~fthenak **********************************************************
