# Resonance Raman¶

In this tutorial we use the vertical gradient Franck-Condon (VG-FC) resonance Raman method to calculate a resonance Raman spectrum in which the incident photon energy is close in energy to one of the excited singlet states of pyrene.

The VG-FC resonance Raman application needs all ground state vibrational frequencies and the electronically excited state gradient of the excited state of interest calculated at the ground state geometry. More information on how resonance Raman is calculated in the VG-FC method can be found in the AMS user manual:

Let us first obtain a pyrene molecule, and optimize its geometry and calculate the vibrational frequencies with DFTB.

**SCM → New Input**. This will open AMSinput.

**Frequencies**checkbox.

**Model → DFTB3**.

**Parameter directory → DFTB.org/3ob-3-1**.

**File → Save As…**and give it the name “pyrene_IR”.

**File → Run**.

**Yes**.

**SCM → Spectra**.

Next we will look at the resonance Raman spectrum in which the incident photon energy is close in energy to the calculated lowest electronically excited singlet state of pyrene. For convenience here we calculate the excited state gradient of the lowest 11 excited states at the ground state geometry, because we want more than 1 dipole-allowed excitation, and the 11th one appears to be an interesting dipole-allowed excitation.

**SCM → New Input**.

**File → Import Coordinates…**and and select the “pyrene_IR.results/dftb.rkf” file.

**Task → Single Point**.

**Model → DFTB3**.

**Parameter directory → DFTB.org/3ob-3-1**.

**Properties → Gradients, Stress Tenor**.

**Nuclear gradients**checkbox.

**Properties → Excitations (UV/Vis)**.

**Type of excitations → Singlet**.

**Number of excitations**.

**Calculate excited state gradients for Excitation number**.

**File → Save As…**and give it the name “pyrene_ES”

**File → Run**.

**SCM → Spectra**.

**Axes → Horizontal Unit → eV**.

**Width → 0.01**.

**Axes → Horizontal Unit → cm-1**.

The dipole allowed excitations are number 1, 2, 5, and 11.
First we calculate a resonance Raman spectrum in which the incident photon energy is close in energy to the calculated lowest excited state at around 26000 cm^{-1}.
For the VG-FC resonance Raman method we need a new input:

**SCM → New Input**.

**File → Import Coordinates…**and and select the “pyrene_ES.ams” file.

**Task → Vibrational Analysis**.

**Model → DFTB3**.

**Parameter directory → DFTB.org/3ob-3-1**.

**Model → Vibrational Analysis**.

**Type → Resonance Raman**.

**Mode file:**and select pyrene_IR.results/dftb.rkf.

**All modes**checkbox.

**Details → Vibrational Analysis Excitation**.

**Excitation file:**and select pyrene_ES.results/dftb.rkf.

**Singlet**.

**Details → Vibrational Analysis Spectrum**.

**Incident Frequency**in cm

^{-1}.

**File → Save As…**and give it the name “pyrene_RR”.

**File → Run**.

**SCM → Spectra**.

^{-1}

The peaks in the spectrum are at frequencies that correspond to fundamental vibrations (IR frequencies), combination bands (sum of different IR frequencies) and overtones (sum of identical IR frequencies). The default Raman order is 2, which means that the summation is over a maximum of 2 IR frequencies. AMSspectra gives a list of frequencies and Raman intensities and the corresponding mode numbers of the IR frequencies involved. For fundamental frequencies only 1 mode number is shown. The mode numbers correspond to the numbers in an energy ordered list of IR frequencies.

Next we calculate a resonance Raman spectrum in which the incident photon energy is close in energy to the calculated 11th excited state at around 40000 cm^{-1}.

**SCM → Input**(go back to the Input window).

**Details → Vibrational Analysis Spectrum**.

**Incident Frequency**in cm

^{-1}.

**File → Run**.

**SCM → Spectra**.

^{-1}