CASTEP > Tasks in CASTEP > Analyzing CASTEP results > Displaying optical properties

# Displaying optical properties

Analysis of optical properties involves two main steps.

## Dielectric function calculation

When a CASTEP calculation returns the matrix elements for electronic interband transitions (Eq. CASTEP 56) in the (hidden) file `.cst_ome`, it is necessary to calculate the dielectric function using Eq. CASTEP 55. This calculation results in the creation of a grid document that contains two sheets. The first is the summary of the calculation that includes the value of the scissors operator, the geometry of the calculation and the smearing applied. The second sheet contains a table of the real and imaginary parts of the dielectric function versus the photon energy (eV).

It is important to set the parameters for this type of calculation correctly in order to be able to compare the results obtained with experimental optical constants.

Scissors operator: This parameter effectively describes the difference between the theoretical and experimental band gap values. When the experimental value is known, you can perform a band structure calculation to find the theoretical band gap. Then simply set the scissors operator to the difference between the two values.

When there is no experimental band gap value but there are data on the dielectric function, you can repeat the calculations with varying scissors operator values and select the one that gives a theoretical absorption edge which matches the experimental value.

In the absence of any experimental data, use a fixed proportion of the theoretical band gap as the scissors operator, for example 25%. This approach allows you to study trends in optical properties across a family of similar compounds.

Calculation: This parameter and the related Polarization or Incidence settings, specify the experimental geometry. These are irrelevant for optically isotropic crystals but they are necessary to study anisotropy of optical properties in crystals with lower symmetry. A good example of the theoretical study of optical anisotropy of crystalline polymers was given by Ambros-Drachl et al. (1995). A more detailed discussion of the meaning of these settings is given in the Optical properties theory topic.

Smearing: This parameter specifies the width of the Gaussian broadening that is used to produce the dielectric function. It is similar to the broadening that is used in density of states calculations.

To create a dielectric function grid

1. Choose Modules | CASTEP | Analysis from the Materials Studio menu bar.
2. Select Optical properties from the list of properties.
3. Use Results file selector to pick the right results file.
4. Select the geometry of the calculation from the Calculation dropdown list.
• If Polarized geometry is selected, specify the Polarization direction in the text boxes.
• If Unpolarized geometry is selected, specify the Incidence direction in the text boxes.
• If Polycrystalline geometry is selected, no additional parameters need to be specified.
5. Specify a Smearing value in the text box (0.5 eV is a good default setting).
6. Click the Calculate button.
7. A new grid document, `seedname Optical Properties.xgd`, is created in the results folder and becomes the active document.

## Display optical constants

The following optical constants can be evaluated based on the calculated dielectric function:

• Reflectivity
• Absorption
• Refractive index, n and k
• Complex optical conductivity
• Energy loss function

Any of these constants and the complex dielectric function itself, can be plotted using Materials Studio. The only additional information that might be required are the Drude correction parameters.

The empirical Drude contribution to the spectra is due to intraband excitations that are not included in the spectra calculated by CASTEP. The Drude contribution, which may be non-zero only for metals and semi-metals, is determined by the DC Conductivity and Drude Damping parameters. Appropriate values for these quantities cover a wide range and are usually taken from experiment. A typical value of the Drude Damping parameter for metals is 0.05 eV. Typical values for the DC Conductivity in metals are around 250 eV. But it must be 0 for insulators. Another common unit for the DC conductivity is 1/(Ohm*cm). The conversion factor between this unit and eV is 1/(Ohm*cm) = 5.924 × 10-4 eV.

To create a chart with optical properties

1. Choose Modules | CASTEP | Analysis from the Materials Studio menu bar.
2. Select Optical properties from the list of properties.
3. Ensure that the grid document, seedname Optical Properties.xgd, is active.
4. Specify the property to be plotted using the Function dropdown list.

If the system is a molecule in the box the only relevant property to display is the Absorption calculated for Polycrystalline geometry.

5. Specify the energy units for the plot using Units dropdown list.
6. Set the Drude correction parameters for metallic systems. Click the More... button to access the Optics Analysis Options dialog.

Ensure that the Drude term is ignored for the nonmetallic systems, i.e. check that the DC Conductivity is 0 on the CASTEP Optical Properties Options dialog.

7. Click the View button.
8. A new chart document, seedname Optical Properties.xcd, is created in the results folder. The summary from the first sheet of the grid document is used as the graph title to assist in identification of the graphs.

The View button is enabled only if a suitable grid document is active.

If the only experimental information available is the value of the static dielectric constant, it is still possible to assess the value of the scissors operator. Calculate the refractive index repeatedly, using different scissors operator values and select the one that gives the best fit between the calculated n(0) value and the square root of the experimental static dielectric constant.