Basis Set Limit CCSD(T) Energies for Extended Molecules via a Reduced-Cost Explicitly Correlated Approach

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Explicitly correlated coupled-cluster methods, particularly CCSD(T), are highly accurate but computationally expensive.
• Existing methods struggle with large molecular systems due to high resource demands.

Purpose of the Study:

• To develop and test approximations to reduce the computational cost of explicitly correlated CCSD(T).
• To enable accurate calculations for larger molecules previously inaccessible.

Main Methods:

• Adaptation of the frozen natural orbital (FNO) technique for explicitly correlated coupled-cluster (CC) methods.
• Application of the natural auxiliary function (NAF) scheme to reduce auxiliary basis set size in density fitting.
• Introduction of the natural auxiliary basis (NAB) approximation to decrease auxiliary basis size for explicitly correlated geminals.

Main Results:

• Achieved speedups of 7x, 5x, and 3x with double-, triple-, and quadruple-ζ basis sets, respectively.
• Demonstrated no loss in accuracy for atomization and reaction energies.
• Enabled accurate calculations for molecules with over 40 atoms within days on modest hardware.

Conclusions:

• The combined FNO-NAF-NAB approach offers significant computational savings for explicitly correlated CCSD(T).
• These approximations make high-accuracy electronic structure calculations feasible for larger and more complex molecular systems.
• The study extends the applicability of explicitly correlated CCSD(T) to a wider range of chemical problems.