Application of SAP-X2C to Spectroscopy and Comparison to Screened Nuclear Spin-Orbit Approximations.

Recently, Surjuse and Valeev [J. Chem. Theory Comput. 22, 3443-3452 (2026)] suggested to use a simple superposition of atomic potentials to account for the two-electron picture-change error in one-electron exact two-component (SAP-X2C) theory. Herein, we generalize this ansatz to analytical derivative theory and apply SAP-X2C to molecular spectroscopy. The accuracy is assessed for NMR, EPR, Mössbauer, UV/vis, as well as X-ray absorption spectroscopy. A thorough comparison with four-component results and the even simpler screened nuclear spin-orbit (SNSO) approximation reveals that both SNSO-X2C and SAP-X2C perform excellently for spectroscopic properties, with SAP-X2C yielding slightly lower errors. Another major advantage is its well-defined thermodynamic limit and less empirical nature. Therefore, SAP-X2C may be expected to become the default choice to mitigate the two-electron picture-change error in density functional theory approaches for spectroscopy, thanks to its accuracy, simplicity, and efficiency. More complicated approaches to account for this error based on atomic mean-field ansätze may still be relevant for high-level correlated methods and highly accurate thermochemistry.