Basis Set Convergence and Extrapolation of Connected Triple Excitation Contributions (T) in Computational Thermochemistry: The W4-17 Benchmark with Up to k Functions

Research output: Chapter in Book/Report/Conference proceedingChapter

Abstract

The total atomization energy of a molecule is the thermochemical cognate of the heat of formation in the gas phase, its most fundamental thermochemical property. We decompose it into different components and provide a survey of them. It emerges that the connected triple excitations contribution is the third most important one, about an order of magnitude less important than the “big two” contributions (mean-field Hartree–Fock and valence CCSD correlation), but 1–2 orders of magnitude more important than the remainder. For the 200 total atomization energies of small molecules in the W4-17 benchmark, we have investigated the basis set convergence of the connected triple excitations contribution (T). Achieving basis set convergence for the valence triple excitations energy is much easier than for the valence singles and doubles correlation energy. Using reference data obtained from spdfghi and spdfghik basis sets, we show that extrapolation from quintuple-zeta and sextuple-zeta yields values within about 0.004 kcal/mol RMS. Convergence to within about 0.01 kcal/mol is achievable with quadruple- and quintuple-zeta basis sets, and to within about 0.05 kcal/mol with triple- and quadruple-zeta basis sets. It appears that radial flexibility in the basis set is more important here than adding angular momenta L: apparently, replacing nZaPa basis sets with truncations of 7ZaPa at L = n gains about one angular momentum for small values of n. We end the article with a brief outlook for the future of accurate electronic structure calculations.
Original languageEnglish
Title of host publicationQuantum Science
Subtitle of host publicationThe Frontier of Physics and Chemistry
PublisherSpringer Nature
Pages467-496
Number of pages30
ISBN (Electronic)9789811944215
ISBN (Print)9789811944208
DOIs
StatePublished - 1 Jan 2022

All Science Journal Classification (ASJC) codes

  • General Chemistry
  • General Physics and Astronomy

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