# TD-CDFT Response Properties For Crystals (OldResponse)¶

BAND can calculate response properties such as the frequency-dependent, dielectric function within the theoretical framework of time-dependent current density function theory (TD-CDFT).

This introductory tutorial will show you how to:

• Set up and run a BAND singlepoint calculation (using ADFJobs and ADFInput)
• Set up and run a BAND TD-CDFT, linear response calculation (using ADFJobs and ADFInput) with the OldResponse method
• Visualize the dielectric funtion using ADFSpectra

If you are not at all familiar with our Graphical User Interface (GUI), check out the Introductory tutorial first.

## Step 1: Create the system¶

We now want to create a silicium-diamond crystal. Let us import the geometry from our database of structures:

2. Switch from ADF to BAND.
3. Search for Silicon in the search box (magnifying glass).
4. Select Crystals → Si.

## Step 2: Run a Singlepoint Calculations (LDA)¶

To get an estimate regarding the error in the band-gap energy of LDA for Si diamond, we shall run a single-point calculation first.

Attention

To enhance the speed of the calculation, we will neglect a convergence study w. r. t. kspace sampling and basis set.

1. Select the ‘BAND Main’ panel.
2. Change Basis set to DZP.
3. Check the Bandstructure and DOS boxes.
4. Go to Details → Integration K-Space.
5. Set the K-Space option for pre-2014 defaults to 3.
6. File → Save As..., use name LDA_SP.adf
7. File → Run

After the calculation finished, we can check the band-gap energy e.g. in the logfile. Furthermore, with the help of the BandStructure module we can validate that a very basic property is reproduced by this rather poor k-space sampling - Si diamond has an indirect band-gap.

8. SCM → Logfile
9. SCM → Bandstructure

## Step 3: Run an OldResponse Calculation (ALDA)¶

We can now start the calculation of the frequency-dependent, dielectric function with the help of linear response TD-CDFT.

In the previous step we learned that the calculated band-gap for the chosen theoretical level is approx. 0.85 eV. This is approximately 0.3 eV below the experimental band-gap. Hence, we will shift the virtual crystal orbitals by this value in energy space. As a reasonable frequency range will sample from 2.0 eV to 5.0 eV with step-size of 0.1 eV.

1. Go to the ‘BAND Main’ panel and uncheck the Bandstructure and DOS boxes.
2. Go to Properties → Dielectric Function (TD-CDFT).
3. Change Method to OldResponse.
4. Change Number of frequencies to 31.
5. Change Starting frequency to 2.0.
6. Change End frequency to 5.0.
7. Change Shift to 0.3.
8. File → Save As..., use name LDA_TDCDFT.adf
9. File → Run

After the calculation finished, we can analyse the calculated dielectric function by plotting it with ADFspectra.

10. SCM → Spectra
11. File → TD-CDFT → Dielectric Function → XX

The general features of the frequency-dependent dielectric function are nicely reproduced, but for a quantitatively better result one has to converge the k-space sampling, basis set and real-space sampling. Also switching to the Berger2015 kernel would improve the results further. [Ref]