  Algebraic diagrammatic construction (ADC) is a computationally efficient
approach for simulating excited electronic states, absorption spectra, and
electron correlation. Due to their origin in perturbation theory, the
single-reference ADC methods may be susceptible to spin contamination when
applied to molecules with unpaired electrons. In this work, we develop an
approach to quantify spin contamination in the ADC calculations of electronic
excitations and apply it to a variety of open-shell molecules starting with
either the unrestricted (UHF) or restricted open-shell (ROHF) Hartree-Fock
reference wavefunctions. Our results show that the accuracy of low-order ADC
approximations (ADC(2), ADC(3)) significantly decreases when the UHF reference
spin contamination exceeds 0.05 a.u. Such strongly spin-contaminated molecules
exhibit severe excited-state spin symmetry breaking that contributes to
decreasing the quality of computed excitation energies and oscillator
strengths. In a case study of phenyl radical, we demonstrate that spin
contamination can significantly affect the simulated UV/Vis spectra, altering
the relative energies, intensities, and order of electronic transitions. The
results presented here motivate the development of spin-adapted ADC methods for
open-shell molecules.
