Orbital spaces in the divide-expand-consolidate coupled cluster method

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster singles and doubles (CCSD) is a high-accuracy quantum chemistry method.
• Calculating large molecular systems with CCSD is computationally intensive.
• Local approximations aim to reduce the computational cost of CCSD.

Purpose of the Study:

• To reinvestigate the theoretical foundation for local CCSD calculations.
• To identify and analyze error sources in independent fragment calculations.
• To develop a robust algorithm for determining necessary orbital spaces for precise calculations.

Main Methods:

• Theoretical analysis of error sources in local CCSD.
• Numerical identification of four distinct error sources.
• Utilizing MP2 (Møller–Plesset perturbation theory) calculations to define local orbital spaces.
• Development and numerical demonstration of an algorithm for orbital space determination.

Main Results:

• Four error sources in local CCSD calculations were identified and analyzed.
• Local orbital spaces for CCSD can be effectively determined from MP2 calculations.
• A novel algorithm precisely identifies orbital spaces for fragment energy calculations.
• The algorithm demonstrates robustness and precision in numerical tests.

Conclusions:

• A solid theoretical foundation for local CCSD calculations on independent fragments is established.
• The developed algorithm enables rigorous error control in divide-expand-consolidate CCSD for large systems.
• This work facilitates accurate and efficient quantum chemical calculations for large molecules.