Deepak, Periodic calculations have several components of the electrostatic energy that are separately infinite, but added they cancel. Instead of dealing with infinites, they are usually eliminated in plane wave codes setting to 0 the G=0 components of the Fourier transforms. I guess that Quantum ESPRESSO satisfies this rule. A component V(G=0) is just the spatial average of V(r), meaning in practice that the electrostatic potential (the Hartree part) is adjusted to zero. The amount of vacuum volume in the supercell represents a weight for the spatial averages. If you change from 10 to 20 angstroms of vacumm width, you are doubling the weight of the vacuum part in the average.
If the average were not adjusted to zero, then I would expect that the average tends softly to cero for infinite vacuum volume, and the Fermi level converge to a constant value. As the average potential is artificially shifted to zero, the Fermi level experiments the opposite shift. For more interesting details, take a look at PRB 77, 115139 (2008). Best regards Eduardo Menendez Proupin Departamento de Qu?mica Fisica Aplicada Facultad de Ciencias Universidad Aut?noma de Madrid 28049 Madrid, Spain Phone: +34 91 497 6706 ---------- Mensaje reenviado ---------- From: Dipak Thakur <[email protected]> To: pw_forum at pwscf.org Date: Tue, 29 Nov 2011 22:05:00 +0530 Subject: [Pw_forum] change in Fermi Energy with ecutoff, Vacuum and degauss Hello every one. I am perplexed by change in Fermi energy with change in ecutoff, Vacuum and degauss. I have done some calculations using a set of ecutoff, Vacuum and degauss and then repeated the same for other set of ecutoff, Vacuum and degauss. I have obtained a value of -0.756 eV for Fermi energy in case where the ecutoff was 60 Ry., Vacuum was 20 A and degauss was 0.02, whereas for the second set with ecutoff of 20 Ry., Vacuum of 10 A and degauss of 0.002 the Fermi energy was 3.0097 eV. Can anyone help me which value of Fermi energy is correct and why? Please help me in understanding the what wrong am I doing? The calculations are related to graphene with Aluminium dopant (one atom of Al and 31 carbon atoms). Thanks and regards Deepak Thakur Research student Swami Ramanand Teerth Marathwada University, Nanded. -- -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20111130/4bf02359/attachment.htm
