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¶
- 1. Start AMSinput.3. Select the “Pyrene (ADF)” entry from the molecules section.6. Select Model → DFTB3.7. Select Parameter directory → DFTB.org/3ob-3-1.8. Click on the ‘Pre-optimize’ button.
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.
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.
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.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.
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.