Reducing Exact Two-Component Theory for NMR Couplings to a One-Component Approach: Efficiency and Accuracy

The self-consistent and complex spin–orbit exact
two-component
(X2C) formalism for NMR spin–spin coupling constants [J. Chem. Theory Comput. 17, 2021, 3874−3994] is reduced to a scalar one-component ansatz.
This way, the first-order response term can be partitioned into the
Fermi-contact (FC) and spin–dipole (SD) interactions as well
as the paramagnetic spin–orbit (PSO) contribution. The FC+SD
terms are real and symmetric, while the PSO term is purely imaginary
and antisymmetric. The relativistic one-component approach is combined
with a modern density functional treatment up to local hybrid functionals
including the response of the current density. Computational demands
are reduced by factors of 8–24 as shown for a large tin compound
consisting of 137 atoms. Limitations of the current ansatz are critically
assessed for Sn, Pb, Pd, and Pt compounds, i.e. the one-component
treatment is not sufficient for tin compounds featuring a few heavy
halogen atoms.