Mixed-order schemes for molecular properties employing ADC/ISR

We investigate mixed-order algebraic diagrammatic construction (ADC) schemes for computing molecular properties employing the intermediate state representation (ISR). In particular, we introduce the ADC(2/1) method, which combines a perturbation theoretical first-order ISR with a second-order ADC excited state calculation. It is benchmarked against the consistent ADC(2/2) method and demonstrates conserved, if not improved, accuracy. In addition, the ISR(1)-d scheme exhibits reduced computational cost compared to an ISR(2) treatment. This substantially extends the applicability of molecular property calculations at the accuracy level of ADC(2) and facilitates algorithmic parallelization, which is essential for ADC(2) calculations of larger molecular systems.