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Abstract: Basis set convergence of correlation effects on molecular atomizationenergies beyond the CCSD coupled cluster with singles and doublesapproximation has been studied near the one-particle basis set limit.Quasiperturbative connected triple excitations, T, converge more rapidly than$L^{-3}$ where $L$ is the highest angular momentum represented in the basisset, while higher-order connected triples, $T 3-T$, converge more slowly -empirically, $\propto L^{-5-2}$. Quasiperturbative connected quadrupleexcitations, Q, converge smoothly as $\propto L^{-3}$ starting with thecc-pVTZ basis set, while the cc-pVDZ basis set causes overshooting of thecontribution in highly polar systems. Higher-order connected quadruples displayonly weak, but somewhat erratic, basis set dependence. Connected quintupleexcitations converge very rapidly with the basis set, to the point where evenan unpolarized double-zeta basis set yields useful numbers. In cases wherefully iterative CCSDTQ5 coupled cluster up to connected quintuplescalculations are not an option, CCSDTQ5 i.e., coupled cluster up toconnected quadruples plus a quasiperturbative connected quintuples correctioncannot be relied upon in the presence of significant nondynamical correlation,whereas CCSDTQ5$ \Lambda$ represents a viable alternative. Connectedquadruples corrections to the core-valence contribution are thermochemicallysignificant in some systems.

. We conclude that ``$3\sigma\leq 1$ kJ-mol-thermochemistry is feasible with current technology, but that the moreambitious goal of $\pm$10 cm$^{-1}$ accuracy is illusory, at least foratomization energies.



Author: Amir Karton, Peter R. Taylor, Jan M. L. Martin

Source: https://arxiv.org/







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