Convergence of coupled cluster perturbation theory

Area of Science:

• Quantum chemistry
• Computational chemistry
• Theoretical chemistry

Background:

• Coupled cluster (CC) methods are vital for accurate electronic structure calculations.
• Perturbation series offer alternative routes to CC energies, but convergence can be problematic.
• Møller-Plesset (MP) perturbation theory is a common benchmark, known for convergence issues.

Purpose of the Study:

• To investigate the convergence behavior of a new family of CC perturbation series.
• To identify factors influencing the convergence of these series, particularly intruder states.
• To compare the convergence of CC perturbation series with standard MP series.

Main Methods:

• Calculated energy corrections for CC perturbation series to high orders.
• Probed for intruder states to determine convergence radii.
• Studied four closed-shell systems (Ne, CH2, HF, F-) using standard and augmented basis sets.

Main Results:

• The choice of the target state, not the parent state, is the primary determinant of series convergence.
• Restricting target excitations (e.g., to triples or quadruples) can mitigate intruder state problems.
• CC perturbation series may diverge with augmented basis sets (diffuse functions), similar to MP series.

Conclusions:

• The target state's nature significantly impacts CC perturbation series convergence.
• While divergences can occur with diffuse basis sets, low-order CC corrections remain practically useful.
• This work provides insights into optimizing CC perturbation series for reliable electronic structure calculations.