Approximate variational coupled cluster theory.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Electronic Structure Theory

Background:

• Variational coupled cluster (VCC) methods offer high accuracy but can be computationally demanding.
• Existing approximations may not fully capture essential electronic correlation effects.
• The need for robust methods applicable to systems with strong static correlation is critical.

Purpose of the Study:

• To develop an accurate and computationally tractable approximation to VCC theory.
• To enhance existing linked pair functional theory for improved electronic structure calculations.
• To ensure the method remains efficient, not exceeding the complexity of traditional coupled cluster approaches.

Main Methods:

• Construction of an approximate VCC method via minimization of a rigorously defined functional.
• Incorporation of low-order corrections to improve accuracy.
• Inclusion of single excitation effects through energy minimization with respect to reference orbitals.

Main Results:

• The proposed method provides an accurate approximation to VCC theory, limited to double substitutions.
• The functional used is rigorously extensive, exact for two-electron systems, and basis set invariant.
• The computational complexity is maintained at or below that of standard coupled cluster methods.

Conclusions:

• The developed approximate VCC theory is a robust method for molecular electronic structure calculations.
• The approach performs well even in the presence of strong static correlation effects.
• This work offers a computationally efficient and accurate alternative for quantum chemistry applications.