Spin-orbit coupling and electron correlation at various coupled-cluster levels for closed-shell diatomic molecules

Area of Science:

• Computational chemistry
• Quantum chemistry
• Spectroscopy

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Heavy-element compounds present unique challenges due to strong spin-orbit coupling (SOC).
• Understanding the interplay between electron correlation and SOC is crucial for predicting molecular properties.

Purpose of the Study:

• To evaluate the performance of the CC2 method for heavy-element compounds.
• To investigate the separability of spin-orbit coupling (SOC) and electron correlation effects.
• To compare different coupled-cluster (CC) levels of theory.

Main Methods:

• Calculations of equilibrium bond lengths and harmonic frequencies.
• Employed coupled-cluster (CC) levels: CCS, CC2, CCSD, and CCSD(T).
• Included spin-orbit coupling (SOC) in the post-Hartree-Fock (HF) step.

Main Results:

• CC2 generally agrees with MP2 for heavy-element compounds, except for SnO, Sb2, PbO, and Bi2.
• CC2 overestimates bond lengths for SnO and PbO by ~0.25 Å.
• SOC effects are significant for Bi2 and At2 at the CCSD(T) level, and for Bi2 at the CCSD level.

Conclusions:

• CC2 is a viable, though not perfect, method for heavy-element compounds.
• SOC effects are substantial and vary with correlation level and specific elements.
• Accurate predictions for heavy-element molecules require careful consideration of both electron correlation and SOC.