[Pw_forum] homo-lumo and conductance

[Gabriele Sclauzero]

ambavale sagar wrote:
> Dear all,
> I am studying ballistic conductance through a molecule attached to Al 
> electrode. 

Including Al surfaces on both sides?

> The k-pt grid used for scf calculation of scattering region 
> made of molecule and few layers of electrode, is 2 2 10 1 1 1. 

If so, this k-point grid looks a bit lousy to me. Unless you use a very large supercell in 
the transverse directions (xy, since z is the transport direction in pwcond), you should 
take more k-points along that directions. Along z, instead, you need less k-points (10 
seems really too much to me). Along z you must include enough slabs before and after the 
junction with the molecule such that the scf potential at the borders of the cell is close 
enough to the scf potential of bulk Al.

You can have a look at this paper in order to get an idea:
Smogunov et al., Phys. Rev. B 78, 014423 (2008)

> I want to 
> see effect of HOMO-LUMO levels of molecule on transmission. Is it 
> sensible to find HOMO-LUMO levels of molecule using separate calculation 
> of isolated molecule with gamma point? How about bandstructure that I 
> got from 20 k-pt calculation of  scattering region periodic in z-direction?

I'm not sure I got your questions. Of course if you study the ballistic conductance 
through the molecule in the tunneling regime, the result will be very sensitive to the 
position of the HOMO and LUMO levels of the molecule. Depending on how strong is the 
binding between the molecule and the electrodes the position and width of these levels 
will be modified more or less by the hybridization with the metal states.
In order to find the position of the HOMO and LUMO in the coupled system you can look at 
the PDOS on the atoms of the molecule (if not too complex) and compare it with the 
corresponding PDOS of the isolated molecule.
Another solution might be to build Wannier functions of the HOMO and LUMO and project onto 
them, but I never tried this solution.

Anyway, you should be careful when trusting DFT results in this kind of system, since 
common XC functionals can mismatch the HOMO-LUMO gap of several eV and there could also be 
problems in getting a reliable alignment with the Fermi level of the metal. Since in 
weakly coupled junctions the conductance is very sensitive to the exact position of the 
molecular levels involved in the tunneling and to the decaying behavior of their tails, 
this issues can lead to  errors of 1 or several orders of magnitude.


Cheers

GS

> 
> Thank you.
> 
> Sagar Ambavale
> PhD student
> M.S. University of Baroda
> India
> 
> ------------------------------------------------------------------------
> Explore and discover exciting holidays and getaways with Yahoo! India 
> Travel Click here! 
> <http://in.rd.yahoo.com/tagline_Travel_1/*http://in.travel.yahoo.com/>
> 
> 
> ------------------------------------------------------------------------
> 
> _______________________________________________
> 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