[Pw_forum] About WanT calculation for large system

[Janiet Wen]

 Dear Prof Marzari and developers :

        Recently, I read  two papers about quantum conductance calculation
based on MLWFs schem for real-space hamiltonian ,i.e. cond-mat/0608146
&PRL95 076804. I did find these two simulations gave quantum conductances
for large systems, .i.e . longest CNT is about 40 nm, which is of thousands
atoms.
     As far as i know, the number of  atoms  that  most  PP-PW codes  can
deal with is ~ 200. So , I think  the hamiltonian for obtaining quantum
conductance of these larger system is not likely to be calculated with PP-PW
method directly , since MLWFs are just post-processing tool for  electronic
structure according to the mannual. There must be some tricks for building
these large systems' real space hamiltonian, though i donot know how to.
Then, I tried to read most original papers about Green functions method and
MLWFs , Below is my understanding of this two simualtions , would you please
give me some comments ?
     For small systems, based on PW calculations ,  we can get full
hamiltonian of the super cell.  Just after distentanglement and localization
procedures, MLWFs are obtained for this supercell.  Then, We can obtain the
real-space hamiltonian using MLWFs as minimal basis set.  With  this
hamiltonian with overlap matrix , we can calculated Quantum conductance of
this system.
    While for lage systems, I think most calculations are similar to the
small system , except the procedure of  real-space hamiltonian buliding .
Since i donot catch the ideas of this step, I just guess that we can bulid
the real-space hamiltonian of large system with  the real-space hamiltonian
of  smaller systems . But i donot know much about this procedure. Would you
please help me out ? Maybe it is trival , but it is really a big barrier
for me to "tunnel ".


  Best wishes,

 Janiet
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