Zero-Field Splitting Parameters from Four-Component Relativistic Methods

TL;DR

This paper introduces a new relativistic computational method to accurately determine zero-field splitting parameters without perturbative spin-orbit treatment, improving robustness especially for actinide complexes.

Contribution

It presents a non-perturbative, multi-state relativistic approach for calculating zero-field splitting parameters using advanced electronic structure theories.

Findings

Comparable accuracy to SI-SO approach on benchmark molecules

Enhanced robustness for actinide complexes

Natural inclusion of direct spin-spin coupling

Abstract

We report an approach for determination of zero-field splitting parameters from four-component relativistic calculations. Our approach involves neither perturbative treatment of spin-orbit interaction nor truncation of the spin-orbit coupled states. We make use of a multi-state implementation of relativistic complete active space perturbation theory (CASPT2), partially contracted N-electron valence perturbation theory (NEVPT2), and multi-reference configuration interaction theory (MRCI), all with the fully internally contracted ansatz. A mapping is performed from the Dirac Hamiltonian to the pseudospin Hamiltonian, using correlated energies and the magnetic moment matrix elements of the reference wavefunctions. Direct spin-spin coupling is naturally included through the full 2-electron Breit interaction. Benchmark calculations on chalcogen diatomics and pseudotetrahedral cobalt(II) complexes show accuracy comparable to the commonly used state-interaction with spin-orbit (SI-SO) approach, while tests on a uranium(III) single-ion magnet suggest that for actinide complexes the strengths of our approach through the more robust treatment of spin-orbit effects and the avoidence of state truncation are of greater importance.