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

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Relativistic Quantum Mechanics

Background:

• Accurate prediction of molecular electronic properties, especially spin-orbit splittings, is crucial for understanding chemical behavior.
• Relativistic effects become significant for heavier elements, necessitating specialized computational frameworks.
• Existing methods may struggle with degeneracies or require computationally expensive unrestricted references for open-shell systems.

Purpose of the Study:

• To implement and validate a relativistic ionization-potential equation-of-motion coupled-cluster (IP-EOMCC) method incorporating up to 3-hole-2-particle (3h2p) excitations.
• To leverage the exact two-component (X2C) framework and the Dirac-Coulomb-Breit Hamiltonian for high accuracy.
• To accurately predict spin-orbit splittings and ionization potentials in open-shell molecules without breaking spatial symmetries.

Main Methods:

• Development of the X2C-IP-EOMCC method with 3h2p excitations.
• Utilizing a closed-shell reference state to avoid degeneracy breaking in open-shell calculations.
• Application to ground and excited states of HCCX+ (X = Cl, Br, I) cations using large basis sets (quadruple-zeta quality).

Main Results:

• The X2C-IP-EOMCC method with 3h2p excitations provides accurate adiabatic ionization potentials (IPs) with a maximum error of ~0.1 eV.
• Calculated spin-orbit splittings show excellent agreement with experimental values, with an accuracy of approximately 0.01 eV.
• Large basis sets and 3h2p correlation effects are essential for achieving high accuracy in absolute energetics.

Conclusions:

• The implemented relativistic X2C-IP-EOMCC method is highly effective for predicting ionization potentials and spin-orbit splittings.
• The use of a closed-shell reference state in this framework circumvents issues with degeneracy breaking in open-shell systems.
• This computational approach offers a reliable tool for studying relativistic effects and electronic properties of heavy-atom containing molecules.