# 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**single point calculation (using**AMSjobs**and**AMSinput**)Set up and run a

**BAND**TD-CDFT, linear response calculation (using**AMSjobs**and**AMSinput**) with the`OldResponse`

methodVisualize the dielectric function using

**AMSspectra**

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 **silicon crystal**. Let us import the geometry from our database of structures:

## Step 2: Run a Single Point Calculation (LDA)¶

We will first perform a single-point calculation of our Silicon crystal using LDA.

Tip

It is good practice to do a convergence study w. r. t. k-space sampling and basis set.

**Basis set**to DZP.

**Bandstructure**and

**DOS**boxes.

**Details → K-Space Integration**

**K-Space grid type**option to

**Symmetric**

**Accuracy**to

**3**.

**File → Save As…**, use name LDA_SP.ams

**File → Run**

After the calculation finished, we can check the band-gap energy e.g. in the logfile. Furthermore, with the help of the **AMSbandstructure** 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.

**SCM → Logfile**

**SCM → Band Structure**

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

We will now calculate the frequency-dependent dielectric function using linear response TD-CDFT.

In the previous step we learned that the calculated band-gap for the chosen theoretical level is 0.76 eV. This is 0.35 eV below the experimental band-gap. Hence, we will shift the virtual crystal orbitals by this value in energy space. We will sample the frequency range from 2.0 eV to 5.0 eV with a step-size of 0.1 eV.

**Bandstructure**and

**DOS**check-boxes.

**Properties → Dielectric Function (TD-CDFT)**.

**Method**to

**OldResponse**.

**Number of frequencies**to

**31**.

**Starting frequency**to

**2.0**.

**End frequency**to

**5.0**.

**Shift**to

**0.35**.

**File → Save As…**, use name LDA_TDCDFT.ams

**File → Run**

After the calculation finished, we can visualize the dielectric function using **AMSspectra**.

**SCM → Spectra**

**Spectra → 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 numerical integration. Also switching to the **Berger2015** kernel would improve the results further. [Ref]