Multireference Algebraic Diagrammatic Construction Theory for Excited States: Extended Second-Order Implementation and Benchmark

We present an implementation and benchmark of new approximations in multireference algebraic diagrammatic construction theory for simulations neutral electronic excitations UV/vis spectra strongly correlated molecular systems (MR-ADC). Following our work on the first-order MR-ADC approximation [J. Chem. Phys. 2018, 149, 204113], we report strict extended second-order methods (MR-ADC(2) MR-ADC(2)-X) that combine description static dynamic electron correlation ground excited states without relying state-averaged reference wave functions. extensive several small molecules, including carbon dimer, ethylene, butadiene. Our results demonstrate that, weakly states, MR-ADC(2) MR-ADC(2)-X outperform third-order single-reference ADC are competitive with from equation-of-motion coupled cluster theory. For character, performance is similar to N-electron valence perturbation In contrast conventional theories, have many attractive features, such as a straightforward efficient calculation excited-state properties direct access outside frontier (active) orbitals.