Two-component relativistic equation-of-motion coupled cluster for electron ionization.

This article focuses on the implementation and assessment of relativistic ionization-potential equation-of-motion coupled-cluster (IP-EOMCC) methods, including IP-EOMCCSD, IP-EOMCCSD(3h2p), and IP-EOMCCSDT, within the molecular mean-field exact two-component (X2C) framework using the full Dirac–Coulomb–Breit (DCB) Hamiltonian. The study applies these methods to the haloacetylene cations HCCX⁺ (X = Cl, Br, I), demonstrating that large, quadruple-ζ-quality basis sets and inclusion of higher-order correlation effects beyond IP-EOMCCSD are necessary for accurate predictions of ionization potentials (IPs) and spin–orbit splittings. Results show that while IP-EOMCCSD with large basis sets overestimates IPs by about 0.2–0.3 eV, composite schemes incorporating IP-EOMCCSDT corrections reduce errors to approximately 0.1 eV or less, closely matching experimental data. Additionally, the one-electron X2C (1eX2C) approach combined with a screened nuclear spin–orbit (SNSO) scaling factor provides computational savings and yields IP and spin–orbit splitting results comparable to the more rigorous DCB-X2C method, especially when using uncontracted basis sets.