Correlation effects beyond coupled cluster singles and doubles approximation through Fock matrix dressing.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster theory is a powerful method for electronic structure calculations.
• Standard coupled cluster with singles and doubles (CCSD) neglects higher excitations, limiting accuracy for some systems.
• Accurately including triple and higher excitations is computationally expensive.

Purpose of the Study:

• To present an accurate single-reference coupled cluster theory that incorporates higher excitations.
• To improve upon conventional coupled cluster with singles and doubles (CCSD) approximations.
• To provide a computationally economical method for capturing correlation effects beyond CCSD.

Main Methods:

• A novel 'dressing' of the Fock operator matrix within a singles and doubles framework.
• The dressing is derived from a second-order perturbative approximation of a similarity transformed Hamiltonian.
• This method induces higher-rank excitations through local renormalization of orbital lines.

Main Results:

• The dressed Fock operator effectively simulates triple and higher excitations.
• The method recovers significant correlation effects beyond the singles and doubles approximation.
• The additional computational cost is economically scaled with n^5.

Conclusions:

• This Fock matrix dressing approach offers a natural improvement over conventional CCSD.
• The method is conceptually simple, easily generalizable to multi-reference schemes, and suitable for strong degeneracy.
• It serves as a lowest-order perturbative approximation to iterative n-body excitation inclusive coupled cluster schemes.