Equation-of-motion coupled-cluster method for ionized states with spin-orbit coupling

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Equation of Motion Coupled-Cluster (EOM-CC) is a powerful extension for excited and ionized states.
• Spin-orbit coupling (SOC) is crucial for understanding heavy element properties and spectroscopy.

Purpose of the Study:

• To implement and evaluate the Equation of Motion Ionized state Coupled-Cluster (EOMIP-CC) approach incorporating spin-orbit coupling (SOC).
• To assess the accuracy of EOMIP-CC with SOC for calculating ionization potentials (IPs) and SOC splittings.
• To investigate the impact of including SOC in different stages of the CC and EOM calculations.

Main Methods:

• Implementation of EOMIP-CC with SOC using a closed-shell reference state.
• Calculation of IPs in the ionized one-particle (1h) and two-particle-one-hole (2p1h) spaces.
• Application of EOMIP-CC at the CC singles (CCS) and CC singles and doubles (CCSD) levels.
• Exploitation of time-reversal and spatial symmetries (D(2h) and subgroups) for computational efficiency.

Main Results:

• The implemented EOMIP-CC approaches provide acceptable estimates for SOC splittings.
• EOMIP-CC with SOC included only in the EOM step yields non-size-intensive IPs.
• EOMIP-CCSD with SOC included only in the EOM step offers reasonable IPs for systems up to the 5th row.
• EOMIP-CCS with SOC in both CC and EOM steps failed to predict a bounded state for I(2)(+).

Conclusions:

• The EOMIP-CC method with SOC is a viable tool for studying ionized states, particularly for heavy elements.
• Care must be taken when using EOMIP-CCS with SOC included in both CC and EOM steps due to potential instabilities.
• The computational efficiency is improved by exploiting molecular symmetries.