Relativistic Douglas-Kroll-Hess calculations of hyperfine interactions within first-principles multireference methods

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Relativistic Quantum Mechanics

Background:

• Accurate prediction of magnetic hyperfine interactions (HFC) is crucial for understanding electronic structures and magnetic properties.
• Relativistic effects become significant for heavier elements, necessitating advanced theoretical treatments.
• Existing nonrelativistic methods often struggle to capture the nuances of HFC in complex systems.

Purpose of the Study:

• To implement a second-order Douglas-Kroll-Hess (DKH) relativistic Hamiltonian for magnetic hyperfine interactions.
• To integrate this into ab initio multireference methods with spin-orbit coupling.
• To calculate relativistic HFC parameters for various atomic and molecular systems, including single-molecule magnets.

Main Methods:

• Implementation of a relativistic magnetic hyperfine interaction Hamiltonian based on DKH theory (2nd order).
• Inclusion of spin-orbit coupling within ab initio multireference methods (Molcas/OpenMolcas).
• Systematic variation of active space size in restricted active space self-consistent field (RAS-SCF) and restricted active space state interaction (RAS-SI) for spin-orbit coupling.

Main Results:

• The DKH relativistic treatment reduces the Fermi contact contribution to HFC, particularly for heavier nuclei.
• Relativistic corrections for Fermi contact contributions show good convergence with increasing active space size.
• Relativistic effects minimally impact the spin-dipole contribution to HFC.
• Accurate HFC parameters were calculated for Tb-based single-molecule magnets (SMMs).
• A significant hyperfine Stark effect was observed in a divalent SMM.

Conclusions:

• The implemented DKH-based relativistic HFC method provides accurate predictions for atomic and molecular systems.
• Relativistic effects are essential for describing Fermi contact interactions, especially in systems with heavy elements and specific electronic configurations.
• The method is applicable to complex systems like SMMs, revealing their electronic properties and tunability.