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 <i>T</i>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 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.