[Pw_forum] transmission calculation

[Manoj Srivastava]

Dear Gabriele,
 Thanks for your response. 
On Thu, 4 Jun 2009, Gabriele Sclauzero wrote:

> Quoting Manoj Srivastava <manoj at phys.ufl.edu>:
> 
> > Dear PWSCF users and developers,
> >  I have been trying to do the conductance calculation for the twin
> > boundary of Cu. I compared my results with previously done calculations
> > and for most K points the transmission coefficients match up very well.
> > But there are some k points that I get transmission coefficients more than
> > 1! This is not reasonable, as the maximum value of transmission
> > coefficient could be 1.I am getting 1.000345, and similar for other k
> > points.
> 
> Which k-points? How many channels are there? Please send me the output  
> file or post the relevant part of it.

 There are quiet a few k points for which transmission is more than 1. 
 For example- 
  k(    3) = (   0.2500000   0.0000000), wk =   0.1666667

 ngper, shell number =          317         133
 ngper, n2d =          317         317
 Nchannels of the left tip =            1
        k1(2pi/a)    k2(2pi/a)       E-Ef (eV)

   0.0634548   0.0000000   0.0000000
 Nchannels of the right tip =            1
        k1(2pi/a)    k2(2pi/a)       E-Ef (eV)

   0.1266351   0.0000000   0.0000000
 to transmit
 T_ij for propagating states:
    1 -->     1   1.0000806
                 1.00008
 Eigenchannel decomposition:
@    1  0.00000  1.00008
                      1.00000
E-Ef(ev), T(x2 spins) =    0.0000000   2.0001613

Also for another k point - 

 k(    5) = (   0.0000000  -0.2500000), wk =   0.1666667

 ngper, shell number =          317         133
 ngper, n2d =          317         317
 Nchannels of the left tip =            1
        k1(2pi/a)    k2(2pi/a)       E-Ef (eV)

   0.0634548   0.0000000   0.0000000
 Nchannels of the right tip =            1
        k1(2pi/a)    k2(2pi/a)       E-Ef (eV)

   0.1266351   0.0000000   0.0000000
 to transmit
 T_ij for propagating states:
    1 -->     1   1.0000806
                 1.00008
 Eigenchannel decomposition:
@    1  0.00000  1.00008
                      1.00000
E-Ef(ev), T(x2 spins) =    0.0000000   2.0001613 

 
> 
> > Do you think transmission coefficients being more than 1 is just
> > numerical error of the code or there is something wrong with my input
> > file? Attached is the input file for lead and scatterign region -
> 
> Transmission coefficients should be between 0 and 1. In the code there  
> are many levels of approximation. Some of them may affect the results.  
> Usually the two relevant parameters one needs to adjust are epsproj  
> and ewind. Why are you tweaking the others? Have you tried the default?
> 
> 
> 
> 
> >     ecutwfc =150.0,
> 
> Why such a high value? You are using US, aren't you?
 I started with low value of ecutwfc, but my transmission was more than 1,
so I started increasing ecutwfc, but still no change. And, yes I am using
USPP.  
> 
> 
> > ATOMIC_POSITIONS
> >   Cu  0.0   0.0                0.0
> >   Cu -0.5   0.288675134        0.81649658
> >   Cu -1.0   0.577350269        1.632993162
> >
> > K_POINTS (automatic)
> > 6 6 8 1 1 1
> > CELL_PARAMETERS {hexagonal}
> >  1.0   0.0    0.0
> > -0.5   0.8660254037844386 0.0
> >  0.0   0.0     2.449489743
> 
> This shouldn't be an issue, but you could try to use ibrav=4 instead  
> of ibrav=0 and specify atomic positions such that all atoms are inside  
> the first primitive cell (i.e. the one with R=0)
 
Setting ibrav=4 or setting ibrav=0 and manually specify unit cell
should give me the same results, I believe.  What is R=0 ?
> 
> 
> 
> 
> 
> >
> >  /
> >   6
> >   0.5000000     -0.5000000               1
> >   4.2584095E-17  0.5000000               1
> 
> Why don't you specify 0.00000 instead? Should not matter anyway.

 Yes, it does not matter. 
> 
> Gabriele Sclauzero
> 
> 
> SISSA & Democritos
> 
> 
> >   0.250000000    0.0000000               1
> >   0.1666667      0.1666667               1
> >  -1.2775231E-16 -0.2500000               1
> >   0.2500000     -0.2500001               1
> > 1
> > 0.0
> >
> >
> > Regards,
> > Manoj Srivastava
> > University of Florida,
> > Gainesville, FL
> >
> > _______________________________________________
> > Pw_forum mailing list
> > Pw_forum at pwscf.org
> > http://www.democritos.it/mailman/listinfo/pw_forum
> >
> 
> 
> 
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Regards, 
Manoj