TD-CDFT Response Properties For Crystals (OldResponse)
******************************************************

BAND Dielectric function Susceptibility Polarizability Refractive
index TD-CDFT

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" method

* Visualize 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:

   1. Open **AMSinput**.
   2. Switch to **BAND**: [image: ADFPanel][image] **→** [image: BANDPanel][image]
   3. Search for **Silicon** in the **search box** [image: Search][image].
   4. Select **Crystals → Si**. (you have to click on the **triangle** to the right of **Crystals** to expand the list)
   5. click on **View → Periodic → Periodic View Type → Repeat Unit Cell**

   [image]


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.

   1. Select the 'BAND Main' panel.
   2. Change **Basis set** to DZP.
   3. Check the **PDOS** and **Band structure** boxes.

   [image]

   4. Go to **Details → K-Space Integration**
   5. Set the **K-Space grid type** option to **Symmetric**
   6. Set the **Accuracy** to **3**.

   [image]

   7. **File → Save As...**, use name LDA_SP.ams
   8. **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.

   9. **SCM → Logfile**
   10. **SCM → Band Structure**

   [image]


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.

   1. Go to the 'BAND Main' panel and un-tick the **Bandstructure** and **DOS** check-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 **0.35**.

   [image]

   8. **File → Save As...**, use name LDA_TDCDFT.ams
   9. **File → Run**

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

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

   [image]

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]
