Relativistic two-component double ionization potential equation-of-motion coupled cluster with the Dirac-Coulomb-Breit Hamiltonian.

We present an implementation of relativistic double ionization potential (DIP) equation-of-motion coupled cluster (EOMCC) with up to 4-hole-2-particle (4h2p) excitations that makes use of the molecular mean-field exact two-component (mmfX2C) framework. We apply mmfX2C-DIP-EOMCC to several neutral atoms and diatomic molecules to obtain the ground and first few excited states of the corresponding dication species, and we observe excellent agreement (to within 0.001 eV) between DIPs obtained from mmfX2C- and four-component DIP-EOMCC calculations that include 3-hole-1-particle (3h1p) excitations, with either the Dirac-Coulomb or Dirac-Coulomb-Gaunt Hamiltonians. We also compare double IPs for mmfX2C-DIP-EOMCC calculations with the full Dirac-Coulomb-Breit Hamiltonian to those from experiment. The mmfX2C-DIP-EOMCC with 3h1p excitations leads to errors in absolute double IPs that generally overestimate experimental data for noble gases by 0.1-0.4 eV, whereas the inclusion of 4h2p excitations results in double IPs that are too low by 0.1-0.2 eV, at the large basis set limit.