Algebraic Diagrammatic Construction Theory of Charged Excitations with Consistent Treatment of Spin–Orbit Coupling and Dynamic Correlation

We present algebraic diagrammatic construction theory for simulating spin-orbit coupling and electron correlation in charged electronic states and photoelectron spectra. Our implementation supports Hartree-Fock and multiconfigurational reference wave functions, enabling efficient correlated calculations of relativistic effects using single-reference (SR-) and multireference-algebraic diagrammatic construction (MR-ADC). We combine the SR- and MR-ADC methods with three flavors of spin-orbit two-component Hamiltonians and benchmark their performance for a variety of atoms and small molecules. When multireference effects are not important, the SR-ADC approximations are competitive in accuracy to MR-ADC, often showing closer agreement with experimental results. However, for electronic states with multiconfigurational character and in nonequilibrium regions of potential energy surfaces, the MR-ADC methods are more reliable, predicting accurate excitation energies and zero-field splittings. Our results demonstrate that the spin-orbit ADC methods are promising approaches for interpreting and predicting the results of modern spectroscopies.