A multireference n-electron Valence State Perturbation Theory study of the electronic spectrum of s-tetrazine

This paper addresses the study of the electronic spectrum of the s-tetrazine molecule in the ab initio frame making use of the multireference n-electron Valence State Perturbation Theory (NEVPT2). The theoretical description of the excited states of this molecule is complex, because of the different computational requirements of the low-lying excited states which must be treated on an equal footing. More than forty electronic excited states of various nature (n -> pi(*), nn -> pi(*)pi(*), pi -> pi(*), n pi -> pi(*)pi(*), and Rydberg) are considered here. Various active spaces are used to reach a good quality zero order description, needed to avoid unrealistic results in the perturbation treatment. The quasi-degenerate perturbation theory must be used in most of the cases, given the presence of a marked mixing among various states. The results here presented have been used to add new information to the interpretation of the experimental spectra. While in many cases previous assignments of the experimental features are confirmed, in various cases they are questioned and new assignments are proposed. With the new assignments, a good agreement is found between experiments and NEVPT2. The comparison with other high level ab initio methods shows that NEVPT2 performs well, being in general in close agreement with Extended-STEOM-CCSD and CC3, while the agreement with CASPT2 and GVVPT2 is less satisfactory (the CASPT2 excitation energies being lower than the NEVPT2 ones by a parts per thousand integral 0.4 eV). Finally, comparing NEVPT2 with TD-DFT, a reasonable accordance with the values obtained with the PBE0 functional is observed, while the agreement with those computed with the HCTH functional is lower.