Unrestricted algebraic diagrammatic construction scheme of second order for the calculation of excited states of medium-sized and large molecules.

An unrestricted version of the algebraic diagrammatic construction (ADC) scheme of the polarization propagator in second order perturbation theory [UADC(2)] is derived via the intermediate state representation. The accuracy of the extended UADC(2)-x approach is evaluated by comparison of computed excitation energies of 11 medium-sized radicals with their corresponding experimental literature values and with excitation energies computed at equation-of-motion-CCSD (coupled clusters singles and doubles) level of theory. Overall, our numerical tests show that UADC(2)-x exhibits an averaged mean deviation in the excitation energies of only 0.3-0.4 eV compared to experimental gas phase data. It provides thus an alternative to coupled-cluster based approaches for the calculation of excited states of medium-sized open-shell molecules.