# [Pw_forum] Electron Phonon interaction: simple-minded questions

Andrea Marini Andrea.Marini at roma2.infn.it
Fri Nov 3 18:38:53 CET 2006

```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

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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/ --

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