Excitation Energies of Canonical Nucleobases Computed by Multiconfigurational Perturbation Theories

In this computational work we assess the performance of ab initio multireference (MR) methods for the calculation of vertical excitation energies of five nucleobases: adenine, guanine, cytosine, thymine and uracil. In total we have studied 38 singlet and 30 triplet excited states. Where possible we used the multireference configuration interaction (MRCI) method as a reference for various flavors of multireference perturbation theory to second order. In particular we have benchmarked CASPT2, NEVPT2 and XMCQDPT2. For CASPT2 we have analyzed the single-state, multistate and extended multistate variants. In addition, we have assessed the effect of the IPEA shift. For NEVPT2 we have used the partially and the strongly contracted variants. Further, we have tested the commonly used RI-CC2, RI-ADC2, and EOM-CCSD methods. Generally, we observe the following trends for singlet excited states: NEVPT2 is the closest MR method to MRCISD+Q, closely followed by CASPT2 with the default IPEA shift. The same trend is observed for triplet states, although NEVPT2 and CASPT2-IPEA are getting nearer. Interestingly, the n,pi* singlet excited states were described more accurately than pi,pi* excited states, while for triplet states the trend is inverted except for NEVPT2. The present work is an important benchmark for future photochemical investigations.