[Wien] optics broken symmetry

[Oleg Rubel]

Dear Wien2k community,

I try to compute opto-elastic properties of InP (zinc-blend structure). 
It is related to a change of the dielectric constant (real part) in 
response to an applied strain. There are no problems with a response to 
a hydrostatic strain, and results agree well with experiments. A problem 
occurs with a uniaxial strain (strained along X-axis only by 0.05%). 
Computed change in the dielectric constant is too large (~ an order of 
magnitude).

Trying to trace back the problem, I did the following:
First, I initialize a tetragonaly-distorted zinc-blend structure 
(init_lapw -b -vxc 19 -ecut -6.5 -numk 800) with the following lattice 
parameters

F   LATTICE,NONEQUIV.ATOMS:  2 

MODE OF CALC=RELA unit=ang 

  11.095785 11.090240 11.090240 90.000000 90.000000 90.000000

Then I set the lattice parameters back to the cubic lattice

F   LATTICE,NONEQUIV.ATOMS:  2 

MODE OF CALC=RELA unit=ang 

  11.090240 11.090240 11.090240 90.000000 90.000000 90.000000

and rerun (x dstart). This allows me to preserve the symmetry of a 
distorted structure (see the structure file below).

Next, I run SCF (run_lapw -ec 0.00001 -cc 0.0001) and optics with 
20x20x20 k-mesh. The results for Re and Im parts of the dielectric 
constant are here:

[oleg at feynman InP-w2k]$ head InP-w2k.epsilon
# 

# Lorentzian broadening with gamma= 0.100000  [eV]
# Im(epsilon) shifted by   0.7860   [eV]
# No intraband contributions added
#
# Energy [eV] Re_eps_xx     Im_eps_xx     Re_eps_yy     Im_eps_yy
#
    0.013610  0.940850E+01  0.988634E-01  0.947674E+01  0.100908E+00
    0.040820  0.940928E+01  0.100340E+00  0.947756E+01  0.102453E+00
    0.068030  0.941084E+01  0.101855E+00  0.947919E+01  0.104042E+00

It seems that the symmetry is broken, which causes later problems with 
opto-elastic coefficients as change of 0.07 in the second decimal point 
of Re_eps for such a small strain is too much.

Once again, there are no problems when the strain tensor does not break 
the zinc-blend cubic symmetry.

Any thoughts are highly appreciated.


Thank you in advance
Oleg

-- 
Oleg Rubel (PhD, PEng)
Department of Materials Science and Engineering
McMaster University
JHE 359, 1280 Main Street West, Hamilton, Ontario L8S 4L8, Canada
Email: rubelo at mcmaster.ca
Tel: +1-905-525-9140, ext. 24094
Web: http://olegrubel.mcmaster.ca

P.S. I run WIEN2k_16.1 (Release 11/17/2016). Our main cluster is down 
for maintenance, so I was not able to check with the newest version of 
Wien2k.

P.P.S. Here is the cubic structure file with a distorted symmetry that I 
run to get the data.

InP 

F   LATTICE,NONEQUIV.ATOMS:  2 

MODE OF CALC=RELA unit=ang 

  11.090240 11.090240 11.090240 90.000000 90.000000 90.000000 

ATOM  -1: X=0.00000000 Y=0.00000000 Z=0.00000000
           MULT= 1          ISPLIT=-2
In         NPT=  781  R0=0.00001000 RMT=    2.0000   Z: 49.000 

LOCAL ROT MATRIX:    0.0000000 0.0000000 1.0000000
                      1.0000000 0.0000000 0.0000000
                      0.0000000 1.0000000 0.0000000
ATOM  -2: X=0.25000000 Y=0.25000000 Z=0.25000000
           MULT= 1          ISPLIT=-2
P          NPT=  781  R0=0.00010000 RMT=    2.0000   Z: 15.000 

LOCAL ROT MATRIX:    0.0000000 0.0000000 1.0000000
                      1.0000000 0.0000000 0.0000000
                      0.0000000 1.0000000 0.0000000
    8      NUMBER OF SYMMETRY OPERATIONS
  1 0 0 0.00000000
  0-1 0 0.00000000
  0 0-1 0.00000000
        1
  1 0 0 0.00000000
  0 0-1 0.00000000
  0-1 0 0.00000000
        2
-1 0 0 0.00000000
  0 1 0 0.00000000
  0 0-1 0.00000000
        3
-1 0 0 0.00000000
  0 0 1 0.00000000
  0-1 0 0.00000000
        4
-1 0 0 0.00000000
  0 0-1 0.00000000
  0 1 0 0.00000000
        5
-1 0 0 0.00000000
  0-1 0 0.00000000
  0 0 1 0.00000000
        6
  1 0 0 0.00000000
  0 0 1 0.00000000
  0 1 0 0.00000000
        7
  1 0 0 0.00000000
  0 1 0 0.00000000
  0 0 1 0.00000000
        8