TD-CDFT Response properties for a 2D periodic system (NewResponse)
******************************************************************

BAND Spectroscopy 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**) for an MoS_2 monolayer

* Set up and run a **BAND** TD-CDFT, linear response calculation
  (using **AMSjobs** and **AMSinput**) with the "NewResponse" method
  for an MoS_2 monolayer

* 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** MoS_2 **monolayer**. 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 **MoS2** in the **search box** [image: Search][image].
   4. Select **Crystals → MoS2_Monolayer**. (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]

If you succeed, your GUI should resemble the following picture:

   [image]


Step 2: Run a Singlepoint Calculations (LDA)
============================================

To get an idea regarding the performance of the LDA XC functional for
the MoS_2 monolayer, we first run a single-point calculation with the
aim of analyzing the resulting band structure. We will already chose
the calculation settings to be the same as for the following linear-
response calculation with TD-CDFT: e.g. the basis set, the numerical
quality, use of symmetry and the sampling of k-space.

   1. Select the 'BAND Main' panel
   2. Change **Basis set** to **DZP**
   3. Change **Numerical Quality** to **Basic**
   4. Tick the **Calculate band structure** and **Calculate PDOS** check-boxes

   [image]

   5. Go to **Details → K-Space Integration** and set **Number of points** to '**11 11**'.

   [image]

   6. Go to **Details → Symmetry** and de-select the check box **Use of symmetry**

   [image]

Now, everything is prepared for the single-point calculation.

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

With the help of the bandstructure we can validate that basic
electronic properties are reproduced by this k-space sampling using
only LDA as XC functional. Unlike the multilayered MoS_2, which has an
indirect band gap, the MoS_2 monolayer has a direct band-gap. (see
also Ref)

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

   [image]


Step 3: Run an NewResponse Calculation (ALDA)
=============================================

We can now start the calculation of the frequency-dependent,
dielectric function using linear response TD-CDFT. As a reasonable
frequency range we shall sample from 1.0 eV to 3.0 eV with a step size
of 0.1 eV.

   1. Go to the 'BAND Main' panel and uncheck the **Calculate band structure** and **Calculate PDOS** boxes.
   2. Go to **Properties → Dielectric Function (TD-CDFT)**.
   3. Change **Method** to **NewResponse**.
   4. Change **Number of frequencies** to **21**.
   5. Change **Starting frequency** to **1.0**.
   6. Change **End frequency** to **3.0**.
   7. Change **Criterion** to **0.1**.
   8. Uncheck the 'z'-box regarding **Components**.

   [image]

Tip:

  The absolute values of the dielectric function depends on the
  assumed two-dimensional volume for the surface. Since this
  definition is arbitrary, you can change the value for the **Volume
  cutoff** at will. The polarizability does not have this arbitrary
  factor.

Since we have already calculated the SCF results in the previous run,
we can restart the SCF from the band.rkf file (this will save some
time during the SCF procedure)

   8. Go to **Details → Restart Details**.
   9. Select the band.rkf file located in the folder **LDA_SP.results**
   10. Tick the checkbox **SCF** next to **Restart**

   [image]

Tip:

  This allows us to split the frequency range into smaller parts
  without too much of an overhead due to the SCF convergence for the
  ground state properties. One could even think about restarting from
  a hybrid DFT calculation with e.g. HSE06.

We are set to run the calculation now:

   11. **File → Save As...**, use name ALDA_TDCDFT.ams
   12. **File → Run**

After the calculation finished, we can analyze the calculated
dielectric function by plotting it with **AMSspectra**.

   13. **SCM → Spectra**
   14. **Spectra → TD-CDFT → Dielectric Function → XX** (you can move the legend with the mouse by drag and drop it to the desired location)

   [image]

With respect to the accuracy of our calculation we can reproduce the
main features of the dielectric function for this system. We cannot
expect the spin-orbit splitting of the first transition at around 1.9
eV to 2.1 eV, since we don't use **Spin-Orbit Relativistic ZORA**.
Currently, it is not possible to use Spin-Orbit coupling with
**NewResponse**.
