Post-CCSD(T) contributions to total atomization energies in multireference systems

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Molecular Electronic Structure Theory

Background:

• Coupled-cluster with single, double, and perturbative triple excitations (CCSD(T)) is a standard for high-accuracy molecular calculations.
• Multireference systems, characterized by significant nondynamical correlation, often require higher-level excitations beyond CCSD(T) for accurate description.
• Basis set convergence is crucial for obtaining reliable results in ab initio electronic structure calculations.

Purpose of the Study:

• To investigate the magnitude and basis set convergence of post-CCSD(T) contributions for a diverse set of 21 first- and second-row molecules.
• To focus on multireference systems where post-CCSD(T) effects are particularly pronounced.
• To explore cost-effective approaches for obtaining high-level correlation contributions using smaller basis sets.

Main Methods:

• Systematic examination of coupled-cluster methods up to CCSDTQ567.
• Calculations performed with various basis sets, including cc-pV6Z, cc-pV5Z, cc-pVTZ, and cc-pVDZ.
• Analysis of basis set convergence and development of extrapolation and scaling factor techniques.

Main Results:

• Post-CCSD(T) contributions were evaluated for systems with moderate to strong nondynamical correlation.
• Extrapolation of perturbative connected quadruples ((Q)) from cc-pVDZ(4s3p1d) and cc-pVTZ basis sets yields near basis-set limit results.
• Combining extrapolated (Q) with iterative Q-(Q) calculations offers a cost-effective route to accurate quadruples contributions.
• Approximations for connected quintuple and sextuple excitations using smaller basis sets were also demonstrated.

Conclusions:

• Cost-effective strategies using smaller basis sets and extrapolation techniques can accurately capture high-level correlation effects.
• These methods significantly improve upon standard basis set extrapolation for post-CCSD(T) contributions.
• The findings provide practical guidance for achieving accurate molecular electronic structure calculations in a computationally efficient manner.