Title: Resolving the notorious case of conical intersections for coupled cluster dynamics,
Abstract: The motion of electrons and nuclei in photochemical events often involve
conical intersections, degeneracies between electronic states. They serve as
funnels for nuclear relaxation - on the femtosecond scale - in processes where
the electrons and nuclei couple nonadiabatically. Accurate ab initio quantum
chemical models are essential for interpreting experimental measurements of
such phenomena. In this paper we resolve a long-standing problem in coupled
cluster theory, presenting the first formulation of the theory that correctly
describes conical intersections between excited electronic states of the same
symmetry. This new development demonstrates that the highly accurate coupled
cluster theory can be applied to describe dynamics on excited electronic states
involving conical intersections.