Core Ionization Potentials (XPS) of thiophene
*********************************************

ADF Spectroscopy XPS Excitations

   [image]

This tutorial demonstrates how to calculate core Ionization Energies
(IE) as probed in X-ray Photoemission Spectroscopy (XPS). After
calculating the IEs of a thiophene molecule, one can assign the
experimental XPS peaks nicely to the different carbon sites in the
molecule. The tutorial is based on the publication C. Grazioli, O.
Baseggio,  M. Stener, G. Fronzoni,  M. de Simone,  M. Coreno, A.
Guarnaccio, A. Santagata, and M. D’Auria, Study of the electronic
structure of short chain oligothiophenes, J. Chem. Phys. 146, 054303
(2017). .


Preparation
===========

Before the computation of IEs, the molecular structure of interest
should be optimized.

Note:

  The geometry used in this tutorial has been relaxed with a BP86/TZP
  DFT calculation.


Setup
=====

Begin by importing the coordinates and setting up the general
parameters for the DFT calculation

   **1.** Download the coordinates "thiophene.xyz" and import them into AMSinput
   **2.** Switch to the [image: ADFPanel][image] and select **Task → Single Point**
   **3.** Check the **Unrestricted** checkbox
   **4.** Select **XC Functional → GGA → BP86**
   **5.** Select **Basis set → TZP**
   **6.** Select **Frozen core → small**
 [image]

Remark: an unrestricted calculation is not needed for a closed shell
molecule, but in this case it is handy, since later we will read the
electron configuration of this calculation and change it to an open
shell electron configuration.

Next, assign a region to one carbon atom to allow specifying a larger
all electron basis set to that atom. The trick used here is that a
frozen core basis set is used on all atoms except the carbon atom of
interest, such that later one can remove 1 electron from the 1s
orbital on this carbon atom, without the risk that during the SCF this
core hole becomes nonlocal, since all other carbon 1s orbitals are in
the frozen core.

   **1.** Switch to the **Model → Regions** panel
   **2.** Select one of the Carbon atoms next to the Sulfur
   **3.** Click on the [image: AddButton][image]
   **4.** Name the region "C1"
 [image]

to define a large basis set for this Carbon atom only:

   **1.** Switch to the panel **Details → Numerical Quality**
   **2.** At the bottom, select **Quality → TZ2P** and **Core → None** for the C1 region.
   **3.** Save and Run the calculation.
 [image]


Results
=======

Once the calculation is finished, open the output (or logfile) and
note the bonding energy.

[image]

Finally, create a hole state and calculate the bonding energy

   **1.** In AMSinput, switch to the panel **Model → Spin and Occupation**
   **2.** Change the occupation of the lowest beta orbital to "0"
   **3.** Save and Run
 [image]

Again, the bonding energy is reported in the output file (or logfile)
once the calculation has finished. The energy difference between the
two bonding energies equals the core Ionization Potential (IP):

   IP(\textrm{C1,1s}) =  E_{hole} - E = 237.5602 -(-53.0143) =
   290.5745 \textrm{ eV [Exp.:290.60 eV]}

Repeating the calculation with one of the Carbon atoms opposite the
Sulfur atoms yields the second IP:

   IP(\textrm{C2,1s}) =  E_{hole} - E = 237.2850 -(-53.0131) =
   290.2981 \textrm{ eV [Exp.:290.35 eV]}

The computed IPs agree very well (probably due to error cancellation)
with the fitted experimental values published in [1]. Note that
typically one is more interested in the difference of the IPs at
different carbon atoms (chemical shifts in carbon 1s IP). Here the
calculated difference is 0.28 eV, which is close to the experimental
value of 0.25 eV.

[1] C. Grazioli1, O. Baseggio,  M. Stener, G. Fronzoni,  M. de Simone,
    M. Coreno, A. Guarnaccio, A. Santagata, and M. D’Auria *Study of
    the electronic structure of short chain oligothiophenes*, J. Chem.
    Phys. 146, 054303 (2017)
