Algebraic DiagrammaticConstruction Theory of ChargedExcitations with Consistent Treatment of Spin–Orbit Couplingand 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.