[Pw_forum] Different TRC will be obtained for the same system?

[Gabriele Sclauzero]

Dear Wang,

Wang Qinjing wrote:
> Dear all:
>  
> I am a new PWSCF user and I was confused by two things.
>  
> One thing is different TRC (Transimission coefficient) at the same 
> energy point will be obtained for the same system. After get the wave 
> functions by the scf calculation, I tried to get the TRC. First I just 
> calculate the TRC at the fermi energy, i.e., the inputfile cond.in for 
> the pwcond.x fiis as follows:
>  
> =========
>     energy0=0.0d0,
>     denergy=0.0d0,
> /
>     1
>     0.0  0.0  1.0
> 1
>   0.0
> =========
> in this way the TRC at fermi energy is about 0.106
>  
> But if I calculate the TRCs for different energies, i.e., the input file 
> for the pwcond.x is as:
>  
> ========
>     energy0=2.0d0,
>     denergy=-0.02d0,
> /
>  1
>     0.0  0.0  1.0
> 201
> ========
> in this way a series of TRC is obtained and the TRC at fermi energy is 
> 5*10^-3. There must be something wrong. Is it a bug? or related to the 
> complie problem?

None of the two. In fact the code performs a basis set reduction of the G_perp vectors 
based on two parameters: ewind and epsproj (see also Doc/INPUT_PWCOND.txt).
The first one selects an energy window starting from the bottom of the band structure up 
to energy0+ewind that will be used to construct the basis set.
As you can notice, the upper limit of this window depends on energy0, i.e. the higher 
scattering energy you want to compute.
Since this basis set can change as you change energy0, also transmission coefficients can.
Of course you want to avoid this, and in order to do this you have to check convergence of 
your complex band structure w.r.t. both ewind and epsproj before computing conductance.
I think that the default values are not enough to achieve convergence: reasonable values 
may be ewind ~ 5 and epsproj ~ 10^-5, but the accuracy and the efficiency in the reduction 
may depend quite a lot on the details of the sistem and of the calculation (so please 
check on your specific system!).
If you want to use the full basis, you can specify ewind=1.d2 or higher (in this case the 
basis reduction will not be used).

Hope this helps,

GS




>  
> Thank you in advance.
>  
> Yours Wang
>  
>  
> ------------------------------------
> 王秦镜，复旦大学物理系研究生
> Wang Qinjing, Physics Department,
> Fudan University
> 
> 
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