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The large and chemically diverse GMTKN55 benchmark was used as a training setfor parametrizing composite wave function thermochemistry protocols akin toG4(MP2)XK theory (Chan et al, JCTC 2019, 15, 4478-4484). Even afterreparametrization, the GMTKN55 WTMAD2 (weighted mean absolute deviation, type2) for G4(MP2)-XK is actually inferior to that of the best rung-4 DFTfunctional, wB97M-V. By increasing the basis set for the MP2 part todef2-QZVPPD, we were able to substantially improve performance at modest cost(if an RI-MP2 approximation is made), with WTMAD2 for this G4(MP2)-XK-D methodnow comparable to the better rung-5 functionals (albeit at greater cost). Athree-tier approach with a scaled MP3/def2-TZVPP intermediate step, however,leads to a G4(MP3)-D method that is markedly superior to even the best doublehybrids wB97M(2) and revDSD-PBEP86-D4. Evaluating the CCSD(T) component with atriple-zeta, rather than split-valence, basis set yields only a modest furtherimprovement that is incommensurate with the drastic increase in computationalcost. G4(MP3)-D and G4(MP2)- XK-D have about 40% better WTMAD2, at similar orlower computational cost, than their counterparts G4 and G4(MP2), respectively:detailed comparison reveals that the difference lies in larger molecules due tobasis set incompleteness error. An E2/ {T,Q} extrapolation and aCCSD(T)/def2-TZVP step provided the G4-T method of high accuracy and with justthree fitted parameters. Using KS orbitals in MP2 leads to the G4(MP3|KS)-Dmethod, which entirely eliminates the CCSD(T) step and has no steps costlierthan scaled MP3; this shows a path forward to further improvements indouble-hybrid density functional methods. G4-T-DLPNO, a variant in whichpost-MP2 corrections are evaluated at the DLPNO- CCSD(T) level, achieves nearlythe accuracy of G4-T but is applicable to much larger systems.

journal of chemical theory and computation

Canonical and DLPNO-based G4(MP2)XK-inspired composite wavefunction  methods parametrized against large and chemically diverse training sets: Are  they more accurate and/or robust than double hybrid DFT?