Characterising conical intersections in DNA/RNA nucleobases with multiconfigurational wave functions of varying active space size

We characterise the photochemically relevant conical intersections between the lowest-lying accessible electronic excited states of the different DNA/RNA nucleobases using Cholesky decomposition-based complete active space selfconsistent field (CASSCF) algorithms. We benchmark two different basis set contractions and several active spaces for each nucleobase and conical intersection type, measuring for the first time how active space size affects conical intersection topographies in these systems, and the potential implications these may have towards their description of photoinduced phenomena. Our results show conical intersection topographies are highly sensitive to the electron correlation included in the model: by changing the amount (and type) of correlated orbitals, conical intersection topographies vastly change, and the changes observed do not follow any converging pattern towards the topographies obtained with the largest and most correlated active spaces. Comparison across systems shows anal- ogous topographies for almost all intersections mediating population transfer to the dark ¹nO/Nπ* states, while no similarities are observed for the “ethylene-like” conical intersection ascribed to mediate the ultrafast decay component to the ground state in all DNA/RNA nucleobases. Basis set size seems to have a minor effect, appearing to be only relevant for purine-based derivatives. We rule out structural changes as a key factor in classifying the different conical intersections, which display almost identical geometries across active space and basis set change, and highlight instead the importance of correctly describing the electronic states involved at these crossing points. Our work shows careful active space selection is essential to accurately describe conical intersection topographies, and therefore to adequately account for their active role in molecular photochemistry.