Dear Gabriele, Thank you very much quick reply. I have some follow up questions on this.
On Tue, 14 Apr 2009, Gabriele Sclauzero wrote: > > Manoj Srivastava wrote: > > Dear PWSCF users and developers, > > I wish to do a transmission calculation and confused about the input > > file. I have a question on example 12 of the package, where transmission > > of monoatomic Al wire with a H atom adsorbed on the side is done. The SCF > > run in the device region is done with (some part of input file is given > > below) > > it is usually called the "scattering region" > > > > > > &system > > ibrav = 6, > > celldm(1) =12.0, > > celldm(3) =1.875, > > > > where the atomic postions of different atoms is > > Al 0.00000000 0.00000000 0.0000 > > Al 0.00000000 0.00000000 0.375 > > Al -0.02779870 0.00000000 .75537515 > > H 0.19269012 0.00000000 .93750000 > > Al -0.02779870 0.00000000 1.11962485 > > Al 0.00000000 0.00000000 1.5 > > > > So, looking at the z coordinate of above system, we notice that device > > region is periodic with period 1.875. > > You are right, the scattering region is a periodic system, since pwscf always > uses PBCs. > The lenght of the scattering region id 12.0*1.875 a.u. and contains all 5 Al > and the H > impurity. But that is exactly the trouble I am having in this. The scattering region should not be periodic like leads, as now we have infinite scattering region! But PWSCF always has PBC, so we should have a large 3rd lattice vector to make the scattering region practically finite. I dont see any super cell here. a_3 is just 1.875*a_0, where a_0 is lattice constant. The atomic postions are all in a_0 unit, which makes me believe that it is a practically infinite system with a_3=1.875*a_0. > > > So, physically we are solving for an > > infinite device region, but in the physical setting of a transmission > > problem leads are semi-infinite and device is finite. Shouldn't we use > > some kind of vacum, i.e. taking 3rd lattice vector large, which > > effectively would represent the finite device region? > > > > Also how much part > > of the leads should be taken as part of device region, > > I don't understand this point. The leads are conceptually different thing > than the > scattering region. The lead is a periodic unit of the "bulk" region (in this > case an > infinitely long monatomic wire) and it is used to compute the generalized > Bloch states, > which in turn are propagated in the scattering region. In the above example, in principle we can have one atom H as scattering region, and Al wire as left and righ leads, but we have taken few Al atoms with H and treated it as scattering region. Thats what I meant by how much part of leads should be taken as scattering region. > > > Is there some kind > > of convergence criterion? Is it like keep increasing part of lead in the > > device reion till further increase does not substantial change device > > behavior, e.g. Bloch's state? > > There is a main convergence criterion (though I don't understand if you are > actually > refering to this). You have to increase the scattering region, adding more Al > atoms in the > wire, such that the complex band structure with real wave-vectors computed > using the > leftmost periodic unit of the wire included in the supercell (the H impurity > being in the > middle of the s.c.) converges to the band structure of an impurity-free wire, > obtained for > instance from a pwscf calculation (or from a pwcond calculation with a 1 atom > cell > containing an Al atom). > > To do this you can use pwcond with > ... > prefixt='prefix of the scattering region' > bdl=ratio between the lenght of the periodic unit and celldm(1) > ikind=0 > band_file='name of file containing the CBS' > ... > > > then compare the real bands (contained in <band_file>.re) with those from > pwscf (obtained > using the 1 atom cell). > > Also convergence of the transmission with the lenght of the scattering region > can be used, > but it is quite more cheap to check convergence of CBS (which can also help > to understand > if everything is going fine), and when the CBS of your "bulk" region (leads) > is correctly > reproduced the transmission should be converged as well. > > HTH > > GS > > > > > > > Regards, > > Manoj Srivastava > > Ph.D. student > > Department of Physics > > University of Florida, Gainesville, FL > > > > > > _______________________________________________ > > Pw_forum mailing list > > Pw_forum at pwscf.org > > http://www.democritos.it/mailman/listinfo/pw_forum > > > > -- > > > o ------------------------------------------------ o > | Gabriele Sclauzero, PhD Student | > | c/o: SISSA & CNR-INFM Democritos, | > | via Beirut 2-4, 34014 Trieste (Italy) | > | email: sclauzer at sissa.it | > | phone: +39 040 3787 511 | > | skype: gurlonotturno | > o ------------------------------------------------ o > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum >