[Pw_forum] Ewald and Coulomb
Let?s consider positive point charges q in one dimensional lattice, that is, at x=a*j, j=-infinity,...,-2,-1,0,1,2,...,+infinity. In order to calculate the electrostatic energy due to Coulomb interaction between the charge at x=0 and all the other charges, one should calculate the summation: 1/a * sum_j q^2/j. However, the summation is not converged and is positive infinite. In summary, electrostatic interaction energy for charged system is infinite and one needs some other technique to calculate it. Yun-Peng From: jiayudai Sent: Saturday, January 14, 2012 10:17 PM To: pw_forum at pwscf.org Subject: Re: [Pw_forum] Ewald and Coulomb Dear Yun-Peng, Thanks for your explanation. In fact, what i mean is that how to treat the ion-ion interactions with some charges. For example, sometimes we want to take out one or more electrons out of the system, thus the tot_charge in the system is not zero. In an extreme case, all electrons are ionized and taken out, there are only positive ions in the system. In this case, the Ewald potential should not be right but the real Coulomb potential should be correct. Since Ewald scheme considers the screnning by the electrons. Thus, i want to use the exact 1/r potential to represent the Ewald scheme. So, how can we reach this goal? Best wishes. Jiayu >>>>>>>>>>>>>>>>>>> what do you mean by "true Coulomb potential"? Based on density functional theory, adding an uniform potential to the system make no difference. In fact, the ion-ion interaction energy is an infinite value because of 1/r type of Coulomb potential. However, if an uniform charge density which makes total charge zero, hence uniform Coulomb potential is added to the system, the electrostatic energy as well as potential is finite, at the same time, physics keep unchanged. best wishes,Yun-Peng Date: Fri, 13 Jan 2012 21:49:59 +0800 From: daiji...@nudt.edu.cn To: pw_forum at pwscf.org Subject: [Pw_forum] Ewald and Coulomb Dear users and developers, Happy new year! I have a confusion about the calculations of ion-ion interactions. We know, we usually use Ewald scheme to represent the real Coulomb potentials in a periodic cell. Generally, it is correct for a neutral system or one electron taken out (or into ) system. However, if the system is constructed with partially charged ions, that is to say, there are more positive charges than negative charges, the Ewald scheme should be not right. Although this system is not stable, but there should be some properties deserved to study. So, how can we calculate the true Coulomb potentials in DFT? That is to say, we do not use Ewald, but only use th 1/r type. I know it can be realized in classical calculations, but i did not find the path to get it in QE. Thanks a lot. Jiayu ___ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120114/b98f04ed/attachment.htm
[Pw_forum] Ewald and Coulomb
Dear Yun-Peng, Thanks for your explanation. In fact, what i mean is that how to treat the ion-ion interactions with some charges. For example, sometimes we want to take out one or more electrons out of the system, thus the tot_charge in the system is not zero. In an extreme case, all electrons are ionized and taken out, there are only positive ions in the system. In this case, the Ewald potential should not be right but the real Coulomb potential should be correct. Since Ewald scheme considers the screnning by the electrons. Thus, i want to use the exact 1/r potential to represent the Ewald scheme. So, how can we reach this goal? Best wishes. Jiayu >>>>>>>>>>>>>>>>>>> what do you mean by "true Coulomb potential"? Based on density functional theory, adding an uniform potential to the system make no difference. In fact, the ion-ion interaction energy is an infinite value because of 1/r type of Coulomb potential. However, if an uniform charge density which makes total charge zero, hence uniform Coulomb potential is added to the system, the electrostatic energy as well as potential is finite, at the same time, physics keep unchanged. best wishes,Yun-Peng Date: Fri, 13 Jan 2012 21:49:59 +0800 From: daiji...@nudt.edu.cn To: pw_forum at pwscf.org Subject: [Pw_forum] Ewald and Coulomb Dear users and developers, Happy new year! I have a confusion about the calculations of ion-ion interactions. We know, we usually use Ewald scheme to represent the real Coulomb potentials in a periodic cell. Generally, it is correct for a neutral system or one electron taken out (or into ) system. However, if the system is constructed with partially charged ions, that is to say, there are more positive charges than negative charges, the Ewald scheme should be not right. Although this system is not stable, but there should be some properties deserved to study. So, how can we calculate the true Coulomb potentials in DFT? That is to say, we do not use Ewald, but only use th 1/r type. I know it can be realized in classical calculations, but i did not find the path to get it in QE. Thanks a lot. Jiayu -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120114/5fae686d/attachment.htm
[Pw_forum] ph.x, q2r.x and low symmetries
Mea Culpa (that's Italian...hmm...actually Latin). It appears that some "leftovers" in temp files persisted between ph.x runs employing "qx" wavevector bases and subsequent "nq" input. As Dick Feynman reportedly observed, " The first principle is that you must not fool yourself, and you are the easiest person to fool." Erase the past...and least what's temporary! -Paul -Original Message- From: pw_forum-bounces at pwscf.org [mailto:pw_forum-boun...@pwscf.org] On Behalf Of W2AGZ Sent: Saturday, January 14, 2012 11:27 AM To: 'PWSCF Forum' Subject: Re: [Pw_forum] ph.x, q2r.x and low symmetries Still working on this issue to make sure (well, as best I can) that I've made no egregiously stupid blunders. The e-p run is quite long...24 IRs...about a day on 8 processors. Should the problem persist, would it be useful to post the input files to ph.x and q2r.x? About 30 lines total? -Paul -Original Message- From: pw_forum-bounces at pwscf.org [mailto:pw_forum-boun...@pwscf.org] On Behalf Of Paolo Giannozzi Sent: Saturday, January 14, 2012 7:47 AM To: PWSCF Forum Subject: Re: [Pw_forum] ph.x, q2r.x and low symmetries On Jan 14, 2012, at 7:59 , W2AGZ wrote: > I'm getting (...) errors after the ph.x step from q2r.x to wit: "nc > already filled: wrong q grid or wrong nr"and, yes, nq = 2 and there is > no xq. > no way to say anyrthing sensible without a (simple!) example P. --- Paolo Giannozzi, Dept of Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222 ___ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum ___ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum
[Pw_forum] query on fermi energy
for the first question, there is no absolute Fermi energy. The only absolute energy is that substracting vacuum energy level from it. best Yun-Peng -- From: Ajit Vallabhaneni Sent: Saturday, January 14, 2012 3:32 PM To: PWSCF Forum Subject: [Pw_forum] query on fermi energy Dear users, 1. I was wondering how to calculate the absolute value of fermi energy for graphene single layer. By varying the size of the super cell in the out of plane direction, i observed that the total energy is constant indicating no interactions in the out of plane direction, but the fermi energy is varying with increasing distance. I think it is the relative fermi energy which is output after the scf calculation. I want to know the absolute value. 2. Another question i have is regarding the e-ph calculation. For a give set of q and k points, what is the best way to ensure that all k+q points are available to avoid the message " q is not a vector in the dense grid" in the calculations"? I observed that using a dense grid is not enough sometimes. Do we have to check it outside of the quantum espresso before the calculation begins? Thanks Ajit ___ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum
[Pw_forum] ph.x, q2r.x and low symmetries
On Jan 14, 2012, at 7:59 , W2AGZ wrote: > I?m getting (...) errors after the ph.x step from q2r.x to wit: ?nc > already filled: wrong q grid or wrong nr?and, yes, nq = 2 and there > is no xq. > no way to say anyrthing sensible without a (simple!) example P. --- Paolo Giannozzi, Dept of Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
[Pw_forum] Ewald and Coulomb
2012/1/14 jiayudai : > Dear Yun-Peng, > > Thanks for your explanation. In fact, what i mean is that how to treat the > ion-ion interactions with some charges. For example, sometimes we want to > take out one or more electrons out of the system, thus the tot_charge in the > system is not zero. In an extreme case, all electrons are ionized and taken > out, there are only positive ions in the system. In this case, the Ewald > potential should not be right but the real Coulomb potential should be > correct. Since Ewald scheme considers the screnning by the electrons. Thus, > i want to use the exact 1/r potential to represent the Ewald scheme. So, how > can we reach this goal? if you want to look at things at this level, then you have to accept the fact that you cannot remove electrons from an infinite system. only from a finite system. you can still do the calculation with a periodic system, if you ignore the divergent term that stems from the total charge of your unit cell. if you do ewald summation with conducting boundary conditions, the forces will not be affected (at least not for the classical system), but the energy is - of course - wrong. either this approach is good enough to you, or you have to use a real space DFT code instead, where you don't have to worry about periodicity by construction (but have some other issues...). axel. > > Best wishes. > > Jiayu > > > > > what do you mean by "true Coulomb potential"? Based on density functional > theory, adding an uniform potential to the system make no difference. In > fact, the ion-ion interaction energy is an infinite value because of 1/r > type of Coulomb potential. However, if an uniform charge density which makes > total charge zero, hence uniform Coulomb potential is added to the system, > the electrostatic energy as well as potential is finite, at the same time, > physics keep unchanged. > best wishes,Yun-Peng > > Date: Fri, 13 Jan 2012 21:49:59 +0800 > > > From: daijiayu at nudt.edu.cn > To: pw_forum at pwscf.org > Subject: [Pw_forum] Ewald and Coulomb > > > Dear users and developers, > > Happy new year! > > > > I have a confusion about the calculations of ion-ion interactions. We know, > we usually use Ewald scheme to represent the real Coulomb potentials in a > periodic cell. Generally, it is correct for a neutral system or one electron > taken out (or into ) system. However, if the system is constructed with > partially charged ions, that is to say, there are more positive charges than > negative charges, the Ewald scheme should be not right. Although this system > is not stable, but there should be some properties deserved to study. > > So, how can we calculate the true Coulomb potentials in DFT? That is to say, > we do not use Ewald, but only use th 1/r type. I know it can be realized in > classical calculations, but i did not find the path to get it in QE. > > > > Thanks a lot. > > > > Jiayu > > > > ___ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > -- Dr. Axel Kohlmeyer akohlmey at gmail.com ?http://goo.gl/1wk0 College of Science and Technology Temple University, Philadelphia PA, USA.
[Pw_forum] scaling of turbo_lanczos.x
2012/1/10 Eduardo Ariel Menendez Proupin > > > a) the itermax parameter to achieve an equivalent resolution of the > spectrum (is the same or must be scaled) > > The itermax depends on the cutoff but not on the number of electrons. If for example in a calculation you use a single or 8 repeated units the total number of iterations to converge is basically the same (but of course the convergence should always be tested in each case). Dario -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120114/252f2f78/attachment.htm
[Pw_forum] query on fermi energy
Yun-peng, Thanks for your response. Is it possible to know what the vaccum energy level is ? I read in the FAQ that it cannot be calculated from the a bulk calculation alone. Thanks Ajit
[Pw_forum] ph.x, q2r.x and low symmetries
Still working on this issue to make sure (well, as best I can) that I've made no egregiously stupid blunders. The e-p run is quite long...24 IRs...about a day on 8 processors. Should the problem persist, would it be useful to post the input files to ph.x and q2r.x? About 30 lines total? -Paul -Original Message- From: pw_forum-bounces at pwscf.org [mailto:pw_forum-boun...@pwscf.org] On Behalf Of Paolo Giannozzi Sent: Saturday, January 14, 2012 7:47 AM To: PWSCF Forum Subject: Re: [Pw_forum] ph.x, q2r.x and low symmetries On Jan 14, 2012, at 7:59 , W2AGZ wrote: > I'm getting (...) errors after the ph.x step from q2r.x to wit: "nc > already filled: wrong q grid or wrong nr"and, yes, nq = 2 and there is > no xq. > no way to say anyrthing sensible without a (simple!) example P. --- Paolo Giannozzi, Dept of Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222 ___ Pw_forum mailing list Pw_forum at pwscf.org http://www.democritos.it/mailman/listinfo/pw_forum
[Pw_forum] query on fermi energy
Dear users, 1. I was wondering how to calculate the absolute value of fermi energy for graphene single layer. By varying the size of the super cell in the out of plane direction, i observed that the total energy is constant indicating no interactions in the out of plane direction, but the fermi energy is varying with increasing distance. I think it is the relative fermi energy which is output after the scf calculation. I want to know the absolute value. 2. Another question i have is regarding the e-ph calculation. For a give set of q and k points, what is the best way to ensure that all k+q points are available to avoid the message " q is not a vector in the dense grid" in the calculations"? I observed that using a dense grid is not enough sometimes. Do we have to check it outside of the quantum espresso before the calculation begins? Thanks Ajit
