Reduced-cost relativistic equation-of-motion coupled cluster method based on frozen natural spinors: A state-specific...

We present the theoretical framework, implementation, and benchmark results for a reduced-cost relativistic equation-of-motion coupled cluster singles and doubles (EOM-CCSD) method based on state-specific frozen natural spinors (SS-FNSs). In this approach, the state-specific frozen natural spinors are derived from the second-order algebraic diagrammatic construction method, providing a compact virtual space for excited-state calculations. The excitation energies computed with the SS-FNS-EE-EOM-CCSD method exhibit smooth convergence with respect to the size of the virtual space and demonstrate significant improvements over those obtained using the conventional MP2-based FNS approach. We have implemented the relativistic SS-FNS-EE-EOM-CCSD method using both the four-component Dirac-Coulomb and the exact two-component atomic mean-field (X2CAMF) Hamiltonians for excitation energies and transition properties. The X2CAMF-based relativistic EOM-CCSD method emerges as a promising approach for large-scale excited-state calculations, achieving excellent agreement with the standard relativistic EOM-CCSD method based on the untruncated canonical spinor basis, but at a significantly reduced computational cost.