A Reduced Cost Equation of Motion Coupled Cluster Method for Excited States Based on State-Specific Natural Orbitals: Theory, Implementation, and Benchmark.

We present a reduced-cost equation-of-motion coupled-cluster method for excited states built on a new state-specific frozen natural orbital (SS-FNO) framework. This approach enables systematic and controllable truncation of the virtual spaces, significantly reducing computational demands while maintaining high accuracy. The method allows for black-box application via two adjustable thresholds and includes a perturbative correction that compensates for truncation errors. We have tested the performance of both the CIS(D) and ADC(2) methods in generating appropriate natural orbitals for excited states. Benchmarking of valence, Rydberg, and charge-transfer excited states demonstrates excellent agreement with canonical EOM-CCSD results, with mean absolute deviations typically below 0.02 eV when ADC(2) natural orbitals with perturbative corrections are applied.