Spin–Orbit Matrix Elements for a Combined Spin-Flip and IP/EA approach

We
present a practical approach for computing the Breit–Pauli
spin–orbit matrix elements of multiconfigurational systems
with both spin and spatial degeneracies based on our recently developed
RAS-nSF-IP/EA method (Houck, S. E.; et al. J. Chem. Theory Comput. 2019, 15, 2278).
The spin–orbit matrix elements over all the multiplet components
are computed using a single one-particle reduced density matrix as
a result of the Wigner–Eckart theorem. A mean field spin–orbit
approximation was used to account for the two-electron contributions.
Basis set dependence as well as the effect of including additional
excitations is presented. The effect of correlating the core and semicore
orbitals is also examined. Surprisingly accurate results are obtained
for spin–orbit coupling constants, despite the fact that the
efficient wave function approximations we explore neglect the bulk
of dynamical correlation.