A classic case of Jahn-Teller effect theory revisited: Ab initio simulation of hyperfine coupling and pseudorotational tunneling in the 12E′ state of Na3

- Multireference and Coupled Cluster methods are applied to Na3.
- The PES is characterized by an analytical function fitted to ab initio data.
- An effective rovibrational Hamiltonian is set up, with all parameters derived ab initio.
- The coupling of pseudorotational tunneling and hyperfine interactions is investigated.
- The theoretical predictions are compared to microwave spectra.

The predictive capabilities of current ab initio approaches are tested in a benchmark study on the well known case of the Na3 ground state. This molecule is small enough to be treated with computationally demanding methods, but also shows an interesting interplay between Jahn-Teller-, spin-orbit-, rovibrational- and hyperfine-interactions. The necessary parameters for the effective Hamiltonian are derived from the potential energy surface of the 12E′ ground state and from spin density evaluations at selected geometries, without any fitting adjustments to experimental data. We compare our results to highly resolved microwave spectra, with the aim to improve previous assignment attempts, where some parameters had to be estimated from fits to measured spectra.