Consistent inclusion of triple substitutions within a coupled cluster based static quantum embedding theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• The MPCC static embedding framework couples high-level coupled cluster (CC) with Møller-Plesset perturbation theory.
• Previous implementations were limited to single and double (SD) substitutions.

Purpose of the Study:

• Extend the MPCC embedding treatment to include triple substitutions for enhanced accuracy in energy differences.
• Develop and assess new formulations, MPCCSDT(pt) and MPCCSDT(it), incorporating triples amplitudes.
• Improve the low-level treatment by including spin fluctuations and charge polarization.

Main Methods:

• Employed a CCSDT solver for the fragment subsystem.
• Constructed perturbative estimates of triples amplitudes for the environment subsystem, accounting for fragment feedback.
• Introduced an iterative treatment for environment triples amplitudes in the MPCCSDT(it) scheme.
• Assessed modified low-level approaches including spin fluctuations and charge polarization.

Main Results:

• Inclusion of triples amplitudes at the fragment level alone is insufficient; perturbative treatment of environment triples is necessary.
• Feedback from environment triples to fragment amplitudes is crucial for challenging molecules like CoH and FeH.
• Second-order perturbative methods for SD amplitudes are needed in some challenging cases, improving upon first-order methods.
• The MP2CCSDT(pt) model offers a favorable balance of cost and accuracy.

Conclusions:

• The extended MPCC framework with triples substitutions significantly improves accuracy for energy differences in quantum chemistry.
• The MP2CCSDT(pt) model is identified as a promising candidate for future computational chemistry applications due to its balance of accuracy and efficiency.
• Further refinements in low-level methods and perturbative treatments are essential for tackling complex chemical systems.