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

We present an implementation of the 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 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 states of the HCCX+ (X = Cl, Br, I) cations, where it is demonstrated that a large basis set (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 ≈0.01 eV, as compared to experimentally obtained values.