Coupling between Thermochemical Contributions of Subvalence Correlation and of Higher-Order Post-CCSD(T) Correlation EffectsA Step toward “W5 Theory”

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 T3–(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 for second-row compounds 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 TAE0 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.