Relativistic Douglas-Kroll-Hess Calculations of Hyperfine Interactions within First Principles Multireference Methods

Hyperfine interaction plays a crucial role in understanding EPR and NMR spectra of materials, provides an important source of decoherence for electronic spins, and enables an external control of nuclear spin qubits in quantum information devices. We present an ab initio method for calculations of relativistic hyperfine coupling parameters using multireference quantum chemistry techniques. The relativistic treatment is based on the second order Douglas-Kroll-Hess (DKH) theory. The method is implemented in the Molcas/Open-Molcas package. The implementation is first tested for atoms, ions, and simple molecules. The results are compared with experiment and other calculations. The effects of relativistic correction, finite nuclei size, core polarization, and dynamic correlations are discussed. Next, we consider hyperfine coupling for single-molecule magnets (SMM)s. Our implementation allows for calculations even for large molecules like TbPc₂ with a well-converged basis. We demonstrate that the relativistic treatment of hyperfine coupling is crucial for SMMs with strong Fermi contact contribution like TbCp₂ molecule with a divalent Tb ion. Finally, we study the electric field dependence of hyperfine interaction in SMMs.