## 4 Conclusions  
In this work, it has been shown that M06-HF, B3 LYP and PBE0 functionals have a similar behaviour for the characterisation of ATZ and photoproducts. For the singlet state of ATZ, it has been shown that the more stable conformer occurs for the _cis_ dihedral angle \(\phi_{1-7-8-9}\) for M06-HF and PBE0, while B3 LYP predicts a bend conformer with \(-124\) deg. Major differences have been found in the singlet electronic states, with M06-HF predicting that the stronger transition may occur for \(S_{0}\to S_{3}\). Such behaviour led to a shift in the simulated spectrum of about 40 nm to a lower wavelength. The remaining functionals described within \(\sim\)10 cm\({}^{-1}\) the UV-Vis spectra of ATZ in methanol. Preliminary studies using equations of motion coupled-cluster theory with the 6-31G basis set, show also a good agreement with experimental spectra. Further studies for basis-set convergence are being carried out to access the quality of the functionals here employed. Other major effects may lead to changes of the overall spectra, since no non-adiabatic effects have been taken into account.  
The proposed mechanism and pathways has been studied both energetically and structurally. In the description of the reaction profile, M06-HF and PBE0 lead to similar energy barriers values, as expected from the available literature. With B3 LYP, the cyclization and the intramolecular proton transfer shows larger energy barriers and the species involved in these steps have lower energy values, a trend also observed in gas-phase studies. The marked changes between the calculated IR spectra of the various species allow to foresee that experimental validation of the mechanism through IR time-resolved spectroscopy is possible after this work, as long as adequate temporal resolution is available.  
From the energetic point of view, the proposed mechanism [19] has been validated, although a more detailed analysis may require the use of molecular dynamical methods and multireference ab initio methods [51]. For the \(S_{n}\) and P\((T_{1})\) states faith, the intersystem crossing, spin-orbit coupling, and conical intersection model will test the rate of conversion towards the \(T_{1}\) state and competing processes, like fast decay to \(S_{1}\), fluorescence, and phosphorescence. The use of an explicit solvation method [52] may also be relevant in the description of the interaction between the solvent and detail interactions with the solvent cage. Drift effects due to the solvent molecules may also change the steric behaviour of the rotations present in the mechanism.  
The following are available online at [[https://www.mdpi.com/article/10.3](https://www.mdpi.com/article/10.3)]([https://www.mdpi.com/article/10.3](https://www.mdpi.com/article/10.3)) 390/app11094045/s1: tables with the geometry, energy, and spectroscopy of the main relevant species.  
The work has been carried out by all authors in a equal basis. All authors have read and agreed to the published version of the manuscript.  
This work has been financed by Portuguese funds through FCT--Fundacao para a Ciencia e a Tecnologia in the framework of the project UIDB/00313/2020.  
The authors declare no conflict of interest.