Until recently, the coupled cluster method was still unable to describe conical intersections—which are central for simulating excited state molecular dynamics. Indeed, the standard coupled cluster method had been conclusively shown to produce severe artifacts at these intersections, such as distorted energy surfaces and unphysical complex-valued energies. This situation changed with the development of the similarity constrained coupled cluster method. In this method, electronic states are constrained to be orthogonal, which thereby restores the proper shape to the energy surfaces and removes any unphysical complex energies. In addition to the energy gradients, also the first-order derivative coupling elements are required to simulate excited state dynamics. Here we describe the implementation of these elements (at the singles and doubles level) and present applications at and near same-symmetry conical intersections.

Nonadiabatic effects with coupled cluster theory

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