Vibronic Spectrum of Pyrazine: New Insights from Multi-state-multi-mode Simulations Parameterized with Equation-of-Motion Coupled-Cluster Methods

This study reports simulations of the lowest band in the electronic absorption spectrum of pyrazine carried out using multi-state-multimode vibronic Hamiltonian parameterized using equation-of-motion coupled-cluster methods. The simulations explain the main spectral features and show how peaks of vibronic nature appear. The most complete vibronic model includes four electronic states and six vibrational modes. The simulations reveal that non-adiabatic coupling with bright states lo- cated as high as 3 eV above the studied state can lead to discernable features in the absorption spectrum. This study demonstrates the power of fully ab initio treat- ments of electronic and vibrational structure and their utility in understanding the mechanisms leading to complex molecular spectra.