Spin-orbit coupling with approximate equation-of-motion coupled-cluster method for ionization potential and electron...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Equation-of-motion coupled-cluster (EOM-CC) methods are crucial for calculating electronic properties like ionization potentials (IPs) and electron affinities (EAs).
• Including spin-orbit coupling (SOC) enhances accuracy for systems with heavy elements but significantly increases computational cost.
• Developing computationally efficient approximations is vital for broader applicability of EOM-CC methods.

Purpose of the Study:

• To investigate and propose approximate cluster amplitude calculation methods for EOM-CC approaches.
• To reduce the computational expense associated with EOM-CC calculations of IPs and EAs, particularly when including SOC.
• To evaluate the accuracy of these approximate methods against full coupled-cluster (CC) calculations.

Main Methods:

• Development and application of approximate EOM-CC methods, specifically CCSD-1 and CCSD-3.
• Comparison of results from approximate methods (CCSD-1, CCSD-3, CC2) with full coupled-cluster singles and doubles (CCSD) calculations.
• Inclusion of spin-orbit coupling (SOC) in post-self-consistent field (SCF) calculations.

Main Results:

• The CCSD-1 approximation provides reasonable IPs and EAs without SOC, comparable to CCSD, and is more economical than CC2.
• The CCSD-3 approximation shows the closest agreement with CCSD for IPs and EAs when SOC is included.
• While CCSD-3 and CCSD-1 show less accurate spin-orbit (SO) splitting, EOM-CC with CC2 generally yields the best agreement with CCSD for SO splitting.

Conclusions:

• Approximate EOM-CC methods, such as CCSD-1 and CCSD-3, can significantly reduce computational cost for calculating IPs and EAs.
• Accurate treatment of SOC effects requires a balanced consideration of both single and double excitation amplitudes.
• CC2 offers a good balance for capturing SO splitting, while CCSD-3 is competitive for IPs and EAs with SOC included.