I here post the following 'again' because my mailer failed to leave the delivered mark on the message.
--- One of the ways to handle charged system in DFT calculation is to use the ESM method. M. Otani and O. Sugino, "First-principles calculations of charged surfaces and interfaces: A plane-wave nonrepeated slab approach," PRB 73, 115407 (2006). It has been implemented in the QE; see the files in the directory: qe-6.8/PW/examples/ESM_example > Besides, another related question is how we define the zero energy in > QE? Energy zero is defined arbitrarily in any DFT calculations. I usually estimate the energy standard (rather than the energy zero) to be the potential energy of vacuum in the slab system. You can estimate it by plotting the x-y averaged potential energies along the z-direction. This function is also implemented in the ESM calculation (check the *.esm file in the working directory of QE). --- best regards kazume NISHIDATE 敬具 西館数芽 [email protected] [email protected] 2021年8月17日(火) 8:52 Andreussi, Oliviero <[email protected]>: > Hi Zhenyao, > > I am writing to give some remarks on simulating negatively charged slabs, > as I am afraid that the correct answer requires to know what the package > “sxdefectalign2d” does exactly (which I don’t). > > A charged slab that is infinitely periodic along x and y must have an > infinite energy. It is not just a matter of interactions with periodic > replicas, even if you remove the periodicity along z, this is not enough to > make your energy finite. If you correct for PBC on the perpendicular > direction, the energy should still be a function of the cell size, as a > uniformly charged slab generates a constant field in space, so the > electrostatic energy of the system is proportional to the cell volume. If > the charged defect is only in the unit cell, but the periodic cells along x > and y have no charged defects, than the energy is finite, but I am not sure > if this is what “sxdefectalign2d” does, as it seems to me kind of tricky to > implement. > > If what you wanted to study is an infinitely periodic charged slab (say > because it is connected to a potential difference), in order for the energy > to be finite and well defined you need to have some compensating charge > somewhere. If you are modeling the interface of a material in an > electrochemical cell, one possibility is to include a model for ionic > diffuse layer in the space above/below your slab. This can be done with > Environ (www.quantum-environ.org), which is a library for modeling > continuum embeddings coupled with QE. > > Standard DFT (e.g. PBE) suffers from self-interaction errors, among other > approximations. As a result, in negative systems you may have that the > extra electron in an anion is not bound, i.e. it’s orbital energy is > greater than zero. Thus, the stable configuration for the extra electron > would be delocalized over the entire cell. If your simulation cell is > small, the free electron states may be slightly higher in energy than your > band energy and the extra electron could still converge, with a positive > band energy. This is what happens in quantum chemistry codes when you use > DFT to simulate anions: having localized basis sets force the extra > electron to stay on the atoms, instead of wandering around as a free > electron. > > I don’t know if this helps, but I would suggest to be particularly careful > with charged slabs. > > Best, > > Oliviero Andreussi > -- > Assistant Professor > Department of Physics > University of North Texas > Email: [email protected] <[email protected]> > Phone: +1-(940)-369-5316 > Skype: olivieroandreussi > Web: https://www.materialab.org > > On Aug 16, 2021, at 4:30 PM, Zhenyao Fang <[email protected]> wrote: > > Dear pwscf users, > > I am performing some calculations on negatively charged slabs (with one > additional electron), and I’m mainly focusing on the energy difference > between charged system and neutral system. With ions fixed, I found that > the charged system has a higher energy than the neutral system, instead of > being lower. > > The energy difference dE = E(q=-1) - E(q=0) is around 3.944 eV. The LUMO > position of the neutral slab is 3.462 eV, and the vacuum level for the > neutral slab is 7.654 eV. Besides, by inspecting the charge difference > plots, the additional charge is localized inside the slab. I understand > that there could be problems about jellium background, so I used the > package “sxdefectalign2d” to correct the interactions due to periodic > images, and the total energy of the charged slab remains the same as the > size of vacuum increases. > > I’m quite confused by the fact that the total energy would increase with > one more electron. Since the additional charge is mainly inside the slab, > it should lower the energy and be stabilised inside the slab. Therefore, I > was wondering how I can reconcile these controversies. Besides, another > related question is how we define the zero energy in QE? In other words, > does LUMO larger or smaller than zero possibly imply the stability of the > system? > > I am looking forward to your replies. Thanks in advance. > > Best, > Zhenyao Fang > > > > > > _______________________________________________ > Quantum ESPRESSO is supported by MaX ( > https://nam04.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.max-centre.eu%2F&data=04%7C01%7Coliviero.andreussi%40unt.edu%7C284d376824a347639a4708d9610df607%7C70de199207c6480fa318a1afcba03983%7C0%7C0%7C637647534859806882%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=aypEIofhzHuvCOgdCS74abOPRfc8uRw859KeLS7Ppvw%3D&reserved=0 > ) > users mailing list [email protected] > > https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Flists.quantum-espresso.org%2Fmailman%2Flistinfo%2Fusers&data=04%7C01%7Coliviero.andreussi%40unt.edu%7C284d376824a347639a4708d9610df607%7C70de199207c6480fa318a1afcba03983%7C0%7C0%7C637647534859806882%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=IwBl%2FV7Kq36drnn%2BivG6G9M%2FNIrtDHD0ZPGrAGcvV9E%3D&reserved=0 > > > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu) > users mailing list [email protected] > https://lists.quantum-espresso.org/mailman/listinfo/users
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