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Algebraic diagrammatic construction (ADC) theory is a computationallyefficient and accurate approach for simulating electronic excitations inchemical systems. However, for the simulations of excited states in moleculeswith unpaired electrons the performance of ADC methods can be affected by thespin contamination in unrestricted Hartree-Fock (UHF) reference wavefunctions.In this work, we benchmark the accuracy of ADC methods for electron attachmentand ionization of open-shell molecules with the UHF reference orbitals(EA/IP-ADC/UHF) and develop an approach to quantify the spin contamination inthe charged excited states. Following this assessment, we demonstrate that thespin contamination can be reduced by combining EA/IP-ADC with the referenceorbitals from restricted open-shell Hartree-Fock (ROHF) or orbital-optimizedM{\o}ller-Plesset perturbation (OMP) theories. Our numerical resultsdemonstrate that for open-shell systems with strong spin contamination in theUHF reference the third-order EA/IP-ADC methods with the ROHF or OMP referenceorbitals are similar in accuracy to equation-of-motion coupled cluster theorywith single and double excitations.

Quantifying and reducing spin contamination in algebraic diagrammatic construction theory of charged excitations