Nonadiabatic dynamics simulation of keto isocytosine: a comparison of dynamical performance of different electronic-structure methods.

The nonadiabatic dynamics of keto isocytosine in the gas phase has been investigated using the on-the-fly trajectory surface hopping method based on two electronic-structure methods: SA-CASSCF and ADC(2). The results estimate an excited-state lifetime of around 1000 fs at the SA-CASSCF level, while a much shorter lifetime of 250-350 fs is obtained at the ADC(2) level. Although three conical intersections (CIs) (Ethyl. I, Ethyl. II and C[double bond, length as m-dash]O stretching) are relevant to the nonadiabatic decay of keto isocytosine, their contributions to the nonadiabatic decay are highly dependent on the electronic-structure methods employed in the dynamics simulation. The Ethyl. II CI is the main channel in the dynamics simulations at the SA-CASSCF level, while the C[double bond, length as m-dash]O stretching CI becomes dominant at the ADC(2) levels. Other high-level electronic-structure methods (MR-CISD and MS-CASPT2) are involved to benchmark our dynamics results. Through the analysis of the reaction pathways from the ground state minimum to the relevant CIs, we expect that the excited-state dynamical features obtained at the MR-CISD and MS-CASPT2 levels should be very similar to those at the SA-CASSCF level. The comparison of results obtained using different excited-state electronic-structure methods could provide guidance for further studies of similar systems.