Vibrationally resolved electronic spectra with DFTB

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:

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 Search and type “pyrene” into the box.
3. Select the “Pyrene (ADF)” entry from the molecules section.
4. Click the Symmetrize button SymmTool to check the symmetry (should be D2h)
5. Select the DFTB panel: ADFPanel → DFTBPanel
6. Select Model → DFTB3.
7. Select Parameter directory → DFTB.org/3ob-3-1.
8. Click on the ‘Pre-optimize’ button.
/scm-uploads/doc.2021/Tutorials/_images/VST_pyrene.png

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. Select Task → Single Point.

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

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

8. Click on File → Save As… and give it the name “pyrene_ES”.
9. Click on File → Run.
10. Wait for the calculation to finish.
11. Click on SCM → Spectra.
12. Axes → Horizontal Unit → eV.
13. Width → 0.01.
/scm-uploads/doc.2021/Tutorials/_images/VST_ES1.png

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… and and select the “pyrene_ES.ams” file.
3. Select the DFTB panel: ADFPanel → DFTBPanel.
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 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.
/scm-uploads/doc.2021/Tutorials/_images/VST1.png

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.
/scm-uploads/doc.2021/Tutorials/_images/VST2.png