[Pw_forum] Electron Phonon interaction: simple-minded questions

[Andrea Marini]

Dear Forum users,

 	I am trying to calculate the el-ph matrix elements of bulk Silicon 
for many bands (let's say 200). The aim is to perform finite temperature 
self-energy calculations.

I have some doubts about my runs, surely due to something I do not fully 
understand.

1. If I do the phonon calculation using a 2x2x2 MP grid PW performs a band 
structure calculation (for each q-point) on the k-points obtained reducing 
by symmetry the 2x2x2 grid + the points linked by a translation of q.

For example at q = ( 0.0000 -1.0000  0.0000 )

                        cart. coord. in units 2pi/a_0
         k(    1) = (   0.0000000   0.0000000   0.0000000), wk =  0.2500000
         k(    2) = (   0.0000000  -1.0000000   0.0000000), wk =  0.0000000
         k(    3) = (   0.5000000  -0.5000000   0.5000000), wk =  1.000000
         k(    4) = (   0.5000000  -1.5000000   0.5000000), wk =  0.0000000
         k(    5) = (   0.0000000  -1.0000000   0.0000000), wk =  0.2500000
         k(    6) = (   0.0000000  -2.0000000   0.0000000), wk =  0.0000000
         k(    7) = (  -1.0000000   0.0000000   0.0000000), wk =  0.5000000
         k(    8) = (  -1.0000000  -1.0000000   0.0000000), wk =  0.0000000

My question is: as I am using a MP grid the shifted points (the ones with 
zero weight) are still points of the 2x2x2 grid (like k(2) and k(5)) . Why 
does PW calculates the states for these points twice ? Is there any way to 
avoid it ?

2. Why does PW performs a band structure calculation for every q-point of 
a subset of the 2x2x2 grid. Can I calculate the states of the whole grid 
once and use it for each q-point phonon calculation ?

3. It seems that my el-ph matrix elements are "strange" in the high energy 
regime, so that I am not able to converge the phononic self-energy as a 
function of the bands used.
The point is that, while I get marvelous Eliashberg functions for 
a metal like Al when I look at  the el-ph matrix elements 
between the fermi surface and high energy bands I'd expect them to go to 
zero as a function of the upper band energy, while it is not. 
Does any of you have experience of the high energy behavior of the el-ph 
matrix elements ?

4. When I try to calculate the matrix elements for many states I get this 
error message already in the sc run

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
      task #         1
      from cgramg1_k : error #         1
      negative or zero norm in S

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

This error disappears when I increase the energy cutoff.

The input file is

  &control
     calculation = 'scf',prefix='Si',
     pseudo_dir = '/home/marini/Sources/espresso/espresso-3.1//pseudo',
     wf_collect=.true.
  /
  &system
     ibrav=  2, celldm(1) =10.183, nat=  2, ntyp= 1,ecutwfc = 15, nbnd = 100
     la2f =.true.
  /
  &electrons
     diago_full_acc=.TRUE.
     diagonalization='david'
     conv_thr = 1.0d-13
     diago_cg_maxiter = 40
  /
ATOMIC_SPECIES
  Si  28.086  Si.vbc.UPF
ATOMIC_POSITIONS
  Si 0.00 0.00 0.00
  Si 0.25 0.25 0.25
K_POINTS {automatic}
   2 2 2 0 0 0

THANKS FOR ANY HELP

Andrea

--------------------------------------------------------------------------
                                Andrea MARINI

        Physics Department, University of Rome "Tor Vergata" (Italy)
               - phone: +39-0672594894 - fax: +39-062023507 -

-- andrea.marini at roma2.infn.it - http://www.fisica.uniroma2.it/~marini/ --