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

TL;DR

This paper develops a relativistic equation-of-motion coupled-cluster method incorporating 3-hole--2-particle excitations within the X2C framework, enabling accurate predictions of ionization potentials and spin-orbit splittings in open-shell molecules.

Contribution

It introduces a novel relativistic EOMCC implementation with 3h2p excitations using the X2C approach, improving accuracy for ionization energies and spin-orbit effects.

Findings

Achieved ~0.1 eV accuracy in ionization potentials.

Predicted spin-orbit splittings within 0.01 eV of experimental values.

Demonstrated necessity of large basis sets and 3h2p correlations.

Abstract

We present an implementation of relativistic ionization-potential (IP) equation-of-motion coupled-cluster (EOMCC) with up to 3-hole--2-particle (3h2p) excitations that makes use of the molecular mean-field exact two-component (mmfX2C) framework and the full Dirac--Coulomb--Breit Hamiltonian. The closed-shell nature of the reference state in an X2C-IP-EOMCC calculation allows for accurate predictions of spin-orbit splittings in open-shell molecules without breaking degeneracies, as would occur in an excitation-energy EOMCC calculation carried out directly on an unrestricted open-shell reference. We apply X2C-IP-EOMCC to the ground and first excited state of the HCCX$^+$ (X = Cl, Br, I) cations, where it is demonstrated that a large basis set (\emph{i.e.}, quadruple-zeta quality) and 3h2p correlation effects are necessary for accurate absolute energetics. The maximum error in calculated adiabatic IPs is on the order of 0.1 eV, whereas spin-orbit splittings themselves are accurate to $\approx 0.01$ eV, as compared to experimentally obtained values.