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The S66 benchmark dataset for noncovalent interactions (NCIs) is studied through localized coupled-cluster methods and general LNO-CCSD(T)-based composite schemes. Very small root-mean-square deviations (≤ 0.05 kcal/mol) for the low-cost composite approaches from the SILVER reference interaction energies of S66 indicate that one can safely avoid carrying out the largest basis set calculations with veryVeryTight thresholds; instead, additivity corrections in smaller basis sets can be applied. Interestingly, the counterpoise corrections do not have an appreciable effect on the composite schemes. These findings may prove useful for intermolecular and intramolecular NCIs of larger systems. [ABSTRACT FROM AUTHOR]

While the inclusion of the nonlocal correlation in fifth rung "double hybrid" functionals is definitely beneficial, one might rightfully ask whether its evaluation in the basis of Kohn-Sham (KS) orbitals has additional value compared to the use of Hartree-Fock reference orbitals (in a type of multilevel scheme). We have investigated this question for a very large and chemically diverse dataset, GMTKN55. We conclude that KS reference orbitals are undoubtedly beneficial, but the benefit is not as large as one might intuitively expect. [ABSTRACT FROM AUTHOR]

We propose here a DFT-based diagnostic for static correlation %TAEX [TPSS@HF – HF] which effectively measures how different the DFT and HF exchange energies for a given HF density are. This and %TAEcorr[TPSS] are two cost-effective a priori estimates for the adequacy of the importance of static correlation. %TAEX[TPSS@HF – HF] contains nearly the same information as the earlier A diagnostic, but may be more intuitive to understand. Principal component and variable clustering analyses of a large number of static correlation diagnostics reveal that much of the variation is explained by just two components, and almost all of it by four; these are blocked by four variable clusters (single excitations; correlation entropy; double excitations; pragmatic energetics). [ABSTRACT FROM AUTHOR]