Problem in optical properties

[abdul_jaleel1]

Dear Users
I want to calculate Frequency dependent dielectric tensor with and without local field effects in RPA of CuO monoclinic phase. Job completed without errors and warning but unable to get dielectric tensor. All data attached.

[merzuk.kaltak]

Dear Abdul,
could you please upload your data in a "zip" format?
I have trouble extracting it with "unrar".
Thanks in advance

[abdul_jaleel1]

attached please.

[merzuk.kaltak]

Dear Abdul,

the dielectric tensor is given in the vasprun.xml file as described in our tutorial on dielectric properties of SiC.
Looking into vasprun.xml you will typically find three sets of data. Please take a look at the tutorial, which also contains a bash script that extracts the data from the file.
For convenience, the very same script is given below:

Code: Select all

awk 'BEGIN{i=1} /imag/,\
                /\/imag/ \
                 {a[i]=$2 ; b[i]=$3 ; i=i+1} \
     END{for (j=12;j<i-3;j++) print a[j],b[j]}' vasprun.xml > imag.dat

awk 'BEGIN{i=1} /real/,\
                /\/real/ \
                 {a[i]=$2 ; b[i]=$3 ; i=i+1} \
     END{for (j=12;j<i-3;j++) print a[j],b[j]}' vasprun.xml > real.dat

# you can use the following line inside the gnuplot terminal to plot the data 
#  plot [0:25] "imag.dat" using (\$1):(\$2) w lp, "real.dat" using (\$1):(\$2) w lp

This script will scrap the file vasprun.xml for lines that start with

Code: Select all

<dielectricfunction comment ...

and will dump the real and imaginary part into "real.dat" and "imag.dat", respectively.

More precisely, following data contains the q->0 limit of the imaginary part of the dielectric tensor
(also known as the HEAD of the microscopic dielectric tensor eps_{G=0,G'=0}(q=0, omega) ).

Code: Select all

<dielectricfunction comment="HEAD OF MICROSCOPIC DIELECTRIC TENSOR (INDEPENDENT PARTICLE)">
   <imag>
    <array>
     <dimension dim="1">gridpoints</dimension>
     <field>energy</field>
     <field>xx</field>
     <field>yy</field>
     <field>zz</field>
     <field>xy</field>
     <field>yz</field>
     <field>zx</field>
     <set> 
      <r>     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
      <r>     0.4627     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
      <r>     0.9250     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
      <r>     1.3866     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
      <r>     1.8472     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
      <r>     2.3065     0.0000     0.0000     0.0000     0.0000     0.0000     0.0000 </r>
         ...   ^          ^          ^          ^          ^          ^          ^   
               |          |          |          |          |          |          |
             energy     eps_xx     eps_yy     esp_zz     esp_xy     eps_yz     eps_xz                   |    

Here the first column of numbers at the bottom are the frequency (energy), followed by the diagonal components xx,yy,zz and the off-diagonal ones xy,yz,zx.

Please note, you might want to increase NBANDS to include more unoccuppied states in your calculation to obtain more reliable results.

[abdul_jaleel1]

I have used the same script to grep real.dat and imag.dat files but unfortunately I have not found the required results. please have a look to attached files.

[SKM]

Dear Admin,

as per the reply below, i am referring to the following;

the first column of numbers at the bottom are the frequency (energy), followed by the diagonal components xx,yy,zz and the off-diagonal ones xy,yz,zx.

Now,I understand that these imaginarydata correspond to the optical absorption spectrum. (hope the understanding is right)

What these xx and yy diagonal components really mean in terms of optical absorption? In other words, if i plot Energy Vs xx or yy, data, then what is the physical meaning (or significance) of these data/plots?

Regards

[merzuk.kaltak]

Dear SKM,
unfortunately we do not have the capacity to answer physics questions.
In short, the individual components of the dielectric tensor can be connected to the absorption and reflection of light from and into a specific direction of the system.
A good explanation of the dielectric tensor can be found in textbooks or even here.

[SKM]

Dear Merzuk
thank you for the reply.
i could see some reference papers about these results of tensor direction of absorption.
However, for obtaining the 'dielectric properties in VASP we must expose the crystal with electric field'. Right?
Now the fundamental query i got is 'In which direction of the crystal the incident Electric field is assumed by VASP?"

Regards
skm

[merzuk.kaltak]

Dear SKM,

To answer you question:
An external electric field is not necessary to determine the dielectric matrix. The same matrix can be determined with density functional perturbation theory (which uses the gradient of the cell-periodic wavefunction). This method is selected in vasp with LEPSILON and solves the Sternheimer equation for a perturbation to arrive at the complete 3x3 dielectric matrix (including other quantities like the Born effective charges).

In contrast, the LCALCEPS method uses three external electric fields to determine the very same dielectric matrix.
By default, the code computes the response of the crystal to an electric field in x, the response of a field in y and finally in z direction to gather the complete 3x3 dielectric response matrix. However, you may also calculate the response of the crystal to one specific electric field with EFIELD_PEAD. This will give you only three components of the dielectric matrix corresponding to the applied field.

Please consider the lecture notes on dielectric properties found on our wiki page.

[SKM]

Thank you
Regards