Ab initio MRSD-CI calculations of the ground and the two lowest-lying excited states of pyrene

The ground and the two lowest-lying excited states of pyrene were studied using π-CASSCF and π-MRSD-CI calculations. The results show that the multireference effect is crucial in predicting the correct order of the two lowest-lying singlet excited states, the 11B2u(S1) and the 11B3u(S2) states. The calculated excitation energies from S0 to S1 and from S0 to S2 are 3.97 and 4.60 eV, respectively, which are about 120% of the experimental values. Various levels of the calculations are presented to analyze the electron correlation effects.