Reduced-cost Relativistic Equation-of-Motion Coupled Cluster Method based on Frozen Natural Spinors: A State-Specific Approach

TL;DR

This paper introduces a cost-effective relativistic EOM-CCSD method using state-specific frozen natural spinors, improving excitation energy calculations and computational efficiency for large-scale excited-state studies.

Contribution

The authors develop and implement a novel relativistic EOM-CCSD approach based on state-specific frozen natural spinors derived from ADC(2), reducing computational cost while maintaining accuracy.

Findings

Significant improvement over MP2-based FNS in excitation energies.

Excellent agreement with standard EOM-CCSD results.

Reduced computational cost for large-scale excited-state calculations.

Abstract

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.