[QE-users] how to extract the matrix elements from the valence bands to conduction bandsat the Brillouin zone center?

[Yu, Jingjing]

Hello everyone,

I am wondering how to extract the matrix elements from the valence bands 
to the conduction bands at the Brillouin zone center, Gamma point in 
Quantum espresso.

I tried to calculate the matrix elements of Si and I got the result in 
the p_avg.dat. But it looks weird, because the the corresponding matrix 
elements along x, y ,z directions (for example, three values |/M/_11 |^2 
along x.y,z directions) are totally different shown below.

*&p_mat nbnd=   6, nks=  29 /
             0.000000  0.000000  0.000000      4
   1
      0.00022045     0.02823071     0.03700090     0.13449672
      0.00458980     0.00203767     0.00400515     0.00964742
   2
      0.00327610     0.02056865     0.04947954     0.00093171
      0.00019941     0.07363126     0.09566180     0.03136617
   3
      0.00146403     0.07631971     0.06399219     0.00701410
      0.00016914     0.00944635     0.02901511     0.16348040*

Because Si has face-centered cubic crystal structure and the matrix 
elements  located at the same positions along x, y, z directions should 
be same, right?

My general calculation procedures are shown below:

*1. run scf.in*

&control
     calculation = 'scf'
     restart_mode='from_scratch'
     prefix = 'si'
     outdir = './outdir'
     pseudo_dir = '/home/yyb/qe-7.0/pseudo'
/
&system
     ibrav=2, celldm(1) =10.410909236,
     nat=2, ntyp=1,
     ecutwfc=40
     ecutrho=320
     nbnd=6
     occupations='smearing', smearing='gaussian',degauss=0.005
     !nosym = .true.
     !noinv = .true.
     !occupations = 'fixed'
     !input_dft = 'gaupbe'
     !nqx1 = 4, nqx2 = 4, nqx3 = 4,
     !x_gamma_extrapolation = .false.
     ! exxdiv_treatment = 'none'
/
&electrons
     conv_thr=1e-10
/
ATOMIC_SPECIES
  Si  28.0855  Si_ONCV_PBE-1.0.upf

ATOMIC_POSITIONS (alat)
  Si 0.00 0.00 0.00
  Si 0.25 0.25 0.25
K_POINTS (automatic)
   8 8 8 0 0 0

*2.run bands.x*

&bands
   outdir='./outdir/'
   prefix='si'
   filband='si.bands.dat'
   lp=.true.
   filp = 'p_avg.dat'
/

I don't know how to get the right result and could anyone who has the 
experience give me some help or suggestions?
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