Influence of the active space on CASSCF nonadiabatic dynamics simulations

Nonadiabatic dynamics simulations are performed at four different extended ab initio CASSCF levels for 4-aminopyrimidine, which had been selected because of its structural relationship to DNA nucleobases, aiming at the assessment of the influence of the active space composition on the qualitative and quantitative dynamics predictions. Reaction pathways connecting the Franck-Condon region to the three main regions of the conical intersection seam were also computed at the same four levels and also using the CASPT2 method. The results show that the theoretical approach has significant influence on quantitative aspects of the dynamics. Nevertheless, relatively uniform qualitative predictions are obtained independently of the CAS level. A well-balanced treatment of lone-pair orbitals in the active space was shown to be a key factor for the reliability of the results.