[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
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
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/ --
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