Coupled cluster theory for nonadiabatic dynamics: nuclear gradients and nonadiabatic couplings in similarity constrained coupled cluster theory

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. We present a few numerical examples that show good agreement with literature values and discuss some limitations of the method. In a separate paper, we show that this implementation can be used in nonadiabatic dynamics, thereby establishing coupled cluster theory as a viable electronic structure method for simulating excited state photochemistry.