<html><head><style type="text/css"><!-- DIV {margin:0px;} --></style></head><body><div style="font-family:times new roman,new york,times,serif;font-size:12pt"><div>Dear Gabriele,<br>Thank you for your reply. I refered to smogunov paper but I could not understand the sentence used there : "Since the system has the supercell (artificial) 2D periodicity in the xy plane, perpendicular to the transport direction, we averaged the transmission over the corresponding 2D BZ using 21 k points in the irreducible part of the BZ.." Are those 21 k-pts belong to xy plane only? what type of grid that may be? 6 6 2 1 1 1 ?<br>In my case supercell dimensions in x and y are 25 a.u. i.e. ~12 ang which includes nearly ~9 ang. of vaccum.<br><br>As you mentioned the limitation of DFT due to xc-functional, can I use DFT calculation to compare qualitatively the Transmission coefficient of same molecule with different geometries?<br><br>would you please mention in
detail how to find positions of HOMO, LUMO in a coupled system?<br></div><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><br>regards<br>Sagar Ambavale<br><div style="font-family: arial,helvetica,sans-serif; font-size: 10pt;"><font size="2" face="Tahoma"><hr size="1"><b><span style="font-weight: bold;"></span></b></font><br>Message: 1<br>Date: Fri, 12 Jun 2009 18:14:46 +0200<br>From: Gabriele Sclauzero <<a ymailto="mailto:sclauzer@sissa.it" href="mailto:sclauzer@sissa.it">sclauzer@sissa.it</a>><br>Subject: Re: [Pw_forum] homo-lumo and conductance<br>To: PWSCF Forum <<a ymailto="mailto:pw_forum@pwscf.org" href="mailto:pw_forum@pwscf.org">pw_forum@pwscf.org</a>><br>Message-ID: <<a ymailto="mailto:4A327EF6.7020206@sissa.it" href="mailto:4A327EF6.7020206@sissa.it">4A327EF6.7020206@sissa.it</a>><br>Content-Type: text/plain; charset=ISO-8859-1; format=flowed<br><br><br>ambavale sagar wrote:<br>> Dear
all,<br>> I am studying ballistic conductance through a molecule attached to Al <br>> electrode. <br><br>Including Al surfaces on both sides?<br><br>> The k-pt grid used for scf calculation of scattering region <br>> made of molecule and few layers of electrode, is 2 2 10 1 1 1. <br><br>If so, this k-point grid looks a bit lousy to me. Unless you use a very large supercell in <br>the transverse directions (xy, since z is the transport direction in pwcond), you should <br>take more k-points along that directions. Along z, instead, you need less k-points (10 <br>seems really too much to me). Along z you must include enough slabs before and after the <br>junction with the molecule such that the scf potential at the borders of the cell is close <br>enough to the scf potential of bulk Al.<br><br>You can have a look at this paper in order to get an idea:<br>Smogunov et al., Phys. Rev. B 78, 014423 (2008)<br><br>> I want to <br>> see effect
of HOMO-LUMO levels of molecule on transmission. Is it <br>> sensible to find HOMO-LUMO levels of molecule using separate calculation <br>> of isolated molecule with gamma point? How about bandstructure that I <br>> got from 20 k-pt calculation of scattering region periodic in z-direction?<br><br>I'm not sure I got your questions. Of course if you study the ballistic conductance <br>through the molecule in the tunneling regime, the result will be very sensitive to the <br>position of the HOMO and LUMO levels of the molecule. Depending on how strong is the <br>binding between the molecule and the electrodes the position and width of these levels <br>will be modified more or less by the hybridization with the metal states.<br>In order to find the position of the HOMO and LUMO in the coupled system you can look at <br>the PDOS on the atoms of the molecule (if not too complex) and compare it with the <br>corresponding PDOS of the isolated
molecule.<br>Another solution might be to build Wannier functions of the HOMO and LUMO and project onto <br>them, but I never tried this solution.<br><br>Anyway, you should be careful when trusting DFT results in this kind of system, since <br>common XC functionals can mismatch the HOMO-LUMO gap of several eV and there could also be <br>problems in getting a reliable alignment with the Fermi level of the metal. Since in <br>weakly coupled junctions the conductance is very sensitive to the exact position of the <br>molecular levels involved in the tunneling and to the decaying behavior of their tails, <br>this issues can lead to errors of 1 or several orders of magnitude.<br><br><br>Cheers<br><br>GS<br><br>> <br>> Thank you.<br>> <br>> Sagar Ambavale<br>> PhD student<br>> M.S. University of Baroda<br>> India<br>> <br>> <br><br><br>o ------------------------------------------------ o<br>| Gabriele Sclauzero, PhD
Student |<br>| c/o: SISSA & CNR-INFM Democritos, |<br>| via Beirut 2-4, 34014 Trieste (Italy) |<br>| email: <a ymailto="mailto:sclauzer@sissa.it" href="mailto:sclauzer@sissa.it">sclauzer@sissa.it</a> |<br>| phone: +39 040 3787 511 |<br>| skype: gurlonotturno |<br>o ------------------------------------------------ o<br><br><br>***************************<br></div></div></div><br>
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