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Accurate quantum chemistry calculations require considering high-level excitations beyond coupled-cluster with single, double, and perturbative triple excitations (CCSD(T)). This study finds cost-effective methods using smaller basis sets to achieve near basis-set limit accuracy for these complex molecular systems.

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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.