hi, in a semi-conductor or insulator there is a energy gap. in this region the dos is zero. so in principle E_f can be placed anywhere in between because E_f is determined by the integral of N(E)dE from zero to E_f which must give the total number of valence electrons. it depends on the conditions (type of broadening & amount of broadening) used in the code. i encountered such situations in wien2k code.
s.auluck > Sorry to bump this up again, but this issue still puzzles me... > > Thanks in advance, > Nicolas > > On Mon, Mar 22, 2010 at 4:02 PM, Nicolas Leconte > <[email protected]>wrote: > >> Hello Alberto, >> >> Thanks for your answer. Comments below... >> >> On Thu, Mar 11, 2010 at 4:12 PM, Alberto Garcia <[email protected]> >> wrote: >> >>> Nicolas, >>> >>> There are two possible reasons for the differences. >>> >>> 1. If the system is an insulator, the placement by Siesta of the Fermi >>> level is rather arbitrary: you can count on it being in the gap, but >>> the actual >>> location will depend on how many states you start with in the >>> bisection algorithm used. Mprop uses a different algorithm. In this >>> case the physics >>> does not depend on the location, as long as it is in the gap. >>> >> >> My system is not an insulator : it's functionnalized graphene with a >> shifted band crossing at the Dirac point (somewhere on the path between >> K' >> and \Gamma). So it is still semi-metallic. >> >> Or does the mere fact I am calculating with SIESTA the wave-functions >> (.WFSX input to be used with mprop) at the \Gamma point only interferes >> somehow in the algorithm? >> >> >>> >>> 2. In general, mprop will work with a certain (gaussian) smearing. >>> Siesta would have used typically a Fermi-Dirac smearing, according to >>> the electronic temperature specified. If you use a more or less >>> equivalent smearing in mprop, you should get similar values for the >>> Fermi level. >>> >> >> If point 1 does not concern me, it should be related to this point. But >> it >> does not fix the issue : >> >> - In the .EIG file of the SIESTA run, my electronic temperature is >> 350 >> K (which corresponds roughly to 0.03 eV). I obtain a Fermi energy >> energy of >> -4.7 eV. >> - In the .stt file after mprop post-processing with the same 0.03 eV >> smearing, I obtain a Fermi energy value of -6.4 eV. >> >> The discrepancy between the two values is still considerable. >> >> Any other comments/ideas? >> >> Thanks in advance, >> Nicolas >> >> >>> Alberto >>> >>> >>> On Thu, Mar 11, 2010 at 9:52 AM, Nicolas Leconte >>> <[email protected]> wrote: >>> > Hello, >>> > Quick question. Is there a particular reason the Fermi energy value >>> obtained >>> > by a regular SIESTA calculation in the *.EIG file is different in >>> absolute >>> > value from the one found in the .stt file after a COOP analysis with >>> the >>> > mprop executable in the Util directory? >>> > Thanks in advance, >>> > Nicolas >>> >> >> > > ....................................................................... Prof. Sushil Auluck Phone:+91-512-6797092/6148 Department of Physics +91-512-6798177(Home) Indian Institute of Technology Cell :+91-9305548667 Kanpur 208016 (UP) Fax :+91-512-6790914 India E-mail:[email protected] ...............................................:[email protected] http://www.iitk.ac.in/phy/People/phy_facvis.html http://www.iitk.ac.in/phy/New01/profile_SA.html ....................................................................... ~ ....................................................................... Prof. Sushil Auluck Phone:+91-512-6797092/6148 Department of Physics +91-512-6798177(Home) Indian Institute of Technology Cell :+91-9305548667 Kanpur 208016 (UP) Fax :+91-512-6790914 India E-mail:[email protected] ...............................................:[email protected] http://www.iitk.ac.in/phy/People/phy_facvis.html http://www.iitk.ac.in/phy/New01/profile_SA.html ....................................................................... ~
