CCSD(T)/CBS atomic and molecular benchmarks for H through Ar.

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of molecular properties is crucial in chemistry.
• Coupled cluster methods, particularly CCSD(T), are highly accurate but computationally expensive.
• Extrapolation to the complete basis set (CBS) limit is necessary for high precision.

Purpose of the Study:

• To compute benchmark correlation energies at the CCSD(T)/CBS limit for a diverse set of 115 chemical species.
• To establish reliable reference data for calibrating approximate computational methods.
• To analyze the convergence patterns of correlation energy components.

Main Methods:

• Extrapolation to the complete basis set (CBS) limit using a sequence of augmented n-tuple-zeta augmented polarization (nZaPa) basis sets (n=4-7).
• Application of the coupled cluster single and double excitation and perturbative triples (CCSD(T)) method.
• Calculation for atoms, ions, diatomic molecules, hydrides, rare gas dimers, polar molecules, and reaction transition states.

Main Results:

• Achieved CCSD(T)/CBS correlation energies for 115 species across the first two rows of the periodic table.
• Demonstrated excellent agreement between calculated CCSD(T) energies and available CCSD-F12b/3C(FIX) values (rms deviation ≤ 0.26 mEh).
• Provided data on the convergence of correlation energy components with increasing basis set size.

Conclusions:

• The computed CCSD(T)/CBS values serve as high-quality benchmarks for quantum chemical calculations.
• These benchmarks can be used to assess and improve the accuracy of smaller basis set approximations.
• The study offers insights into the basis set convergence behavior of electron correlation.