  Coupled cluster theory is one of the most accurate electronic structure
methods for predicting ground and excited state chemistry. However, the
presence of numerical artifacts at electronic degeneracies, such as complex
energies, has made it difficult to apply it in nonadiabatic dynamics
simulations. While it has already been shown that such numerical artifacts can
be fully removed by using similarity constrained coupled cluster (SCC) theory
[J. Phys. Chem. Lett. 2017, 8, 19, 4801-4807], simulating dynamics requires
efficient implementations of gradients and nonadiabatic couplings. Here, we
present an implementation of nuclear gradients and nonadiabatic derivative
couplings at the similarity constrained coupled cluster singles and doubles
(SCCSD) level of theory, thereby making possible nonadiabatic dynamics
simulations using a coupled cluster theory that provides a correct description
of conical intersections between excited states. We present a few numerical
examples that show good agreement with literature values and discuss some
limitations of the method.
