Details of the excited-state potential energy surfaces of adenine by coupled cluster techniques.

EOM-CCSD and CC2-LR methods were used to study the potential energy surfaces of the three lowest excited states (two ππ* and an nπ*) of adenine. The equilibrium structure could only be obtained for the S1 state, which has n-π* character. It was shown that the topology of the coupled cluster surface is such that no minimum for the S2 and S3 states exists in the Franck-Condon region due to conical intersections between these low-lying states. To understand this topology, relevant cuts of the potential energy surfaces have also been calculated, and conical intersections have been located. Even a three-fold intersection between these three states was found. The crossings of these surfaces can be reached from the bright state barrierlessly without major change in the geometry. Therefore, these might play an important role in the ultrafast deactivation of adenine.