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¶
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
Step 3: Vibronic-Structure Tracking¶
For the VG-FC vibronic-structure tracking method we need a new input:
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: