Vibrationally resolved electronic spectra with DFTB
***************************************************

DFTB Spectroscopy Franck-Condon (VG-FC) Vibronic-Structure Tracking
(VST) Excited states

In this tutorial we use the vertical gradient Franck-Condon (VG-FC)
Vibronic-Structure Tracking (VST) method to calculate the
vibrationally resolved absorption spectrum of the first excited
singlet state of pyrene.

There are different methods to calculate a vibrationally resolved
absorption spectrum. Out of these methods VST is the quickest method
and can also be used for much larger sized molecules. It is based on a
mode-tracking algorithm and works by tracking those modes that are
expected to have the largest impact on the vibronic-structure of the
spectrum. More information on VST and related methods can be found in
the AMS user manual:

* Vibrational resolved electronic spectra section of the AMS manual


Step 1: Geometry Optimization
=============================

Let us first obtain a pyrene molecule, and optimize its geometry  with
DFTB.

   **1.** Start AMSinput.
   **2.** In AMSinput, click the search icon [image: Search][image] and type "pyrene" into the box.
   **3.** Select the "Pyrene (ADF)" entry from the molecules section.
   **4.** Click the Symmetrize button [image: SymmTool][image] to check the symmetry (should be D2h)
   **5.** Select the DFTB panel: [image: ADFPanel][image] **→** [image: DFTBPanel][image]
   **6.** Select **Task → Single Point**
   **7.** Select **Model → DFTB3**.
   **8.** Select **Parameter directory → DFTB.org/3ob-3-1**.
   **9.** Click on the 'Pre-optimize' button.
 [image]


Step 2: Excited state gradient
==============================

Here we will look at the vibrationally resolved absorption spectra of
the lowest electronically excited singlet state S1. The VG-FC
Vibronic-Structure Tracking method needs the excited state gradient of
S1 at the ground state geometry.

See also: DFTB manual section on excitations

   **1.** Panel bar **Properties → Gradients, Stress Tensor**.
   **2.** Check the **Nuclear gradients** checkbox.

   **3.** Panel bar **Properties → Excitations (UV/Vis)**.
   **4.** Select **Type of excitations → Singlet**.
   **5.** Enter '1' for **Number of excitations**.
   **6.** Enter '1' for **Calculate excited state gradients for Excitation number**.

   **7.** Click on **File → Save As...** and give it the name "pyrene_ES".
   **8.** Click on **File → Run**.
   **9.** Wait for the calculation to finish.
   **10.** Click on **SCM → Spectra**.
 [image]


Step 3: Vibronic-Structure Tracking
===================================

For the VG-FC vibronic-structure tracking method we need a new input:

   **1.** Click on **SCM → New Input**.
   **2.** Click on **File → Import Coordinates (System) → From File(s)...** and select the "pyrene_ES.ams" file.
   **3.** Select the DFTB panel: [image: ADFPanel][image] **→** [image: DFTBPanel][image].
   **4.** Select **Task → Vibrational Analysis**.
   **5.** Select **Model → DFTB3**.
   **6.** Select **Parameter directory → DFTB.org/3ob-3-1**.

   **7.** Panel bar **Model → Vibrational Analysis**.
   **8.** Select **Type → Vibronic Structure Tracking**.

   **9.** Panel bar **Details → Vibrational Analysis → Vibrational Analysis Excitation**.
   **10.** Click on the folder next to **Excitation file:** and select pyrene_ES.results/dftb.rkf.
   **11.** Enter 'A 1' for **Singlet**.

   **12.** Click on **File → Save As...** and give it the name "pyrene_VST".
   **13.** Click on **File → Run**.
   **14.** Wait for the calculation to finish.
   **15.** Click on **SCM → Spectra**.
 [image]

The spectrum is relative to the 0-0 excitation energy. The default
(artificial) broadening is relatively wide.


Step 4: Increase spectral resolution
====================================

If we want to change the broadening of the vibronic spectrum we can
change the **Line width** in **Details → Vibrational Analysis
Spectrum** and run the calculation again. Here we will also restart
the VST calculation, which saves computation time, for which we need a
new input:

   **1.** Click on **File → Save As...** and give it the name "pyrene_VST_restart".

   **2.** Panel bar **Details → Vibrational Analysis Spectrum**.
   **3.** Enter '50' for **Line width** in cm^-1.

   **4.** Panel bar **Details → Vibrational Analysis Mode Tracking**.
   **5.** Click on the folder next to **VSTrestart file:** and select "pyrene_VST.results/ams.rkf".

   **6.** Click on **File → Run**.
   **7.** Wait for the calculation to finish.
   **8.** Click on **SCM → Spectra**.
 [image]
