Simplified CCSD(T)-F12 methods: theory and benchmarks.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Coupled cluster methods, particularly CCSD(T), are highly accurate but computationally expensive.
• Basis set convergence is a critical factor in achieving reliable computational results.
• Existing approximations often struggle to balance accuracy and computational cost.

Purpose of the Study:

• To provide a detailed explanation and extension of the CCSD(T)-F12x approximations (x = a,b).
• To apply these approximations to open-shell systems.
• To demonstrate significant improvements in basis set convergence for a wide range of chemical properties.

Main Methods:

• Detailed theoretical exposition of CCSD(T)-F12x approximations.
• Extension of the method to handle open-shell electronic systems.
• Extensive benchmark calculations on various chemical properties.

Main Results:

• Demonstrated superior basis set convergence for reaction energies (open- and closed-shell), atomization energies, electron affinities, ionization potentials, geometries, and vibrational frequencies.
• Achieved results exceeding AV5Z quality with AVTZ basis sets, and AVQZ quality with AVDZ basis sets.
• Reported minimal additional computational cost (a few percent) for larger molecules compared to standard CCSD(T).

Conclusions:

• CCSD(T)-F12x approximations offer a significant advancement in computational efficiency and accuracy.
• These methods enable achieving chemical accuracy for total reaction energies using valence-double-zeta basis sets.
• The findings pave the way for more accurate and cost-effective quantum chemical calculations, especially for larger systems.