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-FNS). In this approach, the state-specific frozen natural spinors are derived from the second-order algebraic diagrammatic construction (ADC(2)) 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 truncation threshold 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 to compute 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.
@article{arxiv.2505.07080,
title = {Reduced-cost Relativistic Equation-of-Motion Coupled Cluster Method based on Frozen Natural Spinors: A State-Specific Approach},
author = {Tamoghna Mukhopadhyay and Mrinal Thapa and Somesh Chamoli and Xubo Wang and Chaoqun Zhang and Malaya K. Nayak and Achintya Kumar Dutta},
journal= {arXiv preprint arXiv:2505.07080},
year = {2025}
}