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Coupling between thermochemical contributions of subvalence correlation and of higher-order post-CCSD(T) correlation effects -- a step toward `W5 theory'

Aditya Barman, Gregory H. Jones, Kaila E. Weflen, Margarita Shepelenko, Jan M. L. Martin

TL;DR

This work analyzes how post-CCSD(T) contributions and subvalence correlation influence total atomization energies for first- and second-row molecules, with emphasis on systems containing adjacent second-row atoms. It benchmarks and extends W4-family thermochemistry toward a Weizmann-5 (W5) protocol, employing high-level basis sets, explicit post-CCSD(T) corrections, and careful treatment of open-shell geometries via ROCCSD(T). The results reveal substantial subvalence post-CCSD(T) effects in second-row species, demonstrate the synergy between T3-(T) and Q contributions, and show improved agreement with ATcT data for many key species, including recent boron, silicon, and sulfur compounds. The findings emphasize the need for accurate geometry and core-valence treatment in second-row thermochemistry and propose practical extrapolation strategies that balance computational cost with accuracy, advancing toward a more predictive W5 framework.

Abstract

We consider the thermochemical impact of post-CCSD(T) contributions to the total atomization energy (TAE, the sum of all bond energies) of first- and second-row molecules, and specifically their coupling with the subvalence correlation contribution. In particular, we find large contributions from (Q) when there are several neighboring second-row atoms. Otherwise, both higher-order triples $T_3$--(T) and connected quadruples (Q) are important in systems with strong static correlation. Reoptimization of the reference geometry for core-valence correlation increases the calculated TAE across the board, most pronouncedly so for second-row compounds with neighboring second-row atoms. %just slightly increases the calculated TAE for all species, but more pronouncedly so if strong static correlation is present, as well as for second-row compounds, again especially with neighboring second-row atoms. We present a first proposal for a `W5 theory' protocol and compare computed TAEs for the W4-08 benchmark with prior reference values. For some key second-row species, the new values represent nontrivial revisions. Our predicted TAE$_0$ values (TAE at 0 K) agree well with the ATcT (active thermochemical tables) values, including for the very recent expansion of the ATcT network to boron, silicon, and sulfur compounds.

Coupling between thermochemical contributions of subvalence correlation and of higher-order post-CCSD(T) correlation effects -- a step toward `W5 theory'

TL;DR

This work analyzes how post-CCSD(T) contributions and subvalence correlation influence total atomization energies for first- and second-row molecules, with emphasis on systems containing adjacent second-row atoms. It benchmarks and extends W4-family thermochemistry toward a Weizmann-5 (W5) protocol, employing high-level basis sets, explicit post-CCSD(T) corrections, and careful treatment of open-shell geometries via ROCCSD(T). The results reveal substantial subvalence post-CCSD(T) effects in second-row species, demonstrate the synergy between T3-(T) and Q contributions, and show improved agreement with ATcT data for many key species, including recent boron, silicon, and sulfur compounds. The findings emphasize the need for accurate geometry and core-valence treatment in second-row thermochemistry and propose practical extrapolation strategies that balance computational cost with accuracy, advancing toward a more predictive W5 framework.

Abstract

We consider the thermochemical impact of post-CCSD(T) contributions to the total atomization energy (TAE, the sum of all bond energies) of first- and second-row molecules, and specifically their coupling with the subvalence correlation contribution. In particular, we find large contributions from (Q) when there are several neighboring second-row atoms. Otherwise, both higher-order triples --(T) and connected quadruples (Q) are important in systems with strong static correlation. Reoptimization of the reference geometry for core-valence correlation increases the calculated TAE across the board, most pronouncedly so for second-row compounds with neighboring second-row atoms. %just slightly increases the calculated TAE for all species, but more pronouncedly so if strong static correlation is present, as well as for second-row compounds, again especially with neighboring second-row atoms. We present a first proposal for a `W5 theory' protocol and compare computed TAEs for the W4-08 benchmark with prior reference values. For some key second-row species, the new values represent nontrivial revisions. Our predicted TAE values (TAE at 0 K) agree well with the ATcT (active thermochemical tables) values, including for the very recent expansion of the ATcT network to boron, silicon, and sulfur compounds.
Paper Structure (16 sections, 2 figures, 5 tables)

This paper contains 16 sections, 2 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Computational methods
  3. A remark concerning reference geometries
  4. Results and Discussion
  5. ROCCSD(T) vs. UCCSD(T) geometries
  6. Effect of CV geometry shift on thermochemistry
  7. CCSD and (T) basis set extrapolation
  8. Effect of subvalence post-CCSD(T) on thermochemistry
  9. Revisiting valence post-CCSD(T) contributions
  10. Post-CCSDT(Q)$_\Lambda$ contributions
  11. While we are at it: scalar relativistics and DBOC reconsidered
  12. Rotational zero-point energy
  13. A first attempt at a W5 protocol; comparison with earlier results
  14. Comparison for W4-08 with earlier W4-17 results and ATcT
  15. A remark on some atomic heats of formation
  16. ...and 1 more sections

Figures (2)

  • Figure 1: For the P2 diatomic in the cc-pwCVTZ basis set; (a) Left-hand pane: dependence of different valence energy components (hartree) on the displacement (Å) from the CCSD(T)/pwCVQZ reference bond distance $r_e$; (b) Right-hand pane: same graph for the subvalence contributions.
  • Figure 2: Table of Contents Graphic