Zero-Field Splitting Parameters from Four-Component Relativistic Methods.

Area of Science:

• Quantum Chemistry
• Relativistic Calculations
• Spectroscopy

Background:

• Zero-field splitting (ZFS) parameters are crucial for understanding magnetic properties of molecules.
• Accurate determination of ZFS often requires sophisticated computational methods.
• Existing methods may involve approximations in treating spin-orbit interactions.

Purpose of the Study:

• To develop and validate a new computational approach for determining zero-field splitting parameters.
• To avoid perturbative treatments and state truncation in spin-orbit calculations.
• To assess the method's performance across different molecular systems.

Main Methods:

• Utilized multi-state relativistic perturbation theories (CASPT2, NEVPT2) and multi-reference configuration interaction (MRCI).
• Employed a fully internally contracted ansatz with a mapping from the Dirac to pseudospin Hamiltonian.
• Incorporated direct spin-spin coupling via the Breit interaction.

Main Results:

• Achieved accuracy comparable to the state-interaction with spin-orbit (SI-SO) method for benchmark systems.
• Demonstrated superior performance for actinide complexes due to robust spin-orbit treatment.
• Validated the approach on chalcogen diatomics and cobalt(II) complexes.

Conclusions:

• The proposed method provides an accurate and robust alternative for ZFS parameter determination.
• It is particularly advantageous for systems with strong spin-orbit coupling, like actinides.
• The avoidance of state truncation enhances reliability in relativistic calculations.