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