Treatment of spin–orbit coupling with internally contracted multireference coupled cluster theory

We present the formalism for computing state-interaction matrix elements within the multistate extension of internally contracted multireference coupled-cluster theory. In particular, we focus on the determination of spin-orbit coupling matrix elements. A pilot implementation is presented and tested for the zero-field splitting of atomic ²P and ³P terms as well as for molecular ²Π terms. We also investigate the impact of the non-Hermiticity of the underlying coupled-cluster theory on spin-orbit couplings between non-equivalent states. For this, we consider the coupling between the ³Σ^- ground state and the ¹Π excited state of NH for several interatomic distances. Only a very small asymmetry is found for this case, but it is also seen that the present theory has to be carefully revisited with respect to the treatment of the response of the reference coefficients in order to avoid artefacts.