A simple composite scheme is presented, based on a combination of density functional
geometry and frequencies evaluation, valence energies obtained using the CCSD(T)-f12 method
extrapolated to the complete basis set limit, and core-valence correlation corrections employing
the MP2 method. The procedure was applied to the 38 reactions in Truhlar’s HTBH38/08 and
NHTBH38/08 databases and the errors in the barriers with respect to their best values are
presented. Mean unsigned deviation (MUD) for the complete set of 68 independent barriers is 0.40
kcal mol-1
, compared to 1.31 kcal/mol for G4 and 1.62 kcal/mol for the dispersion-corrected M06-
2X method. The accuracy of the procedure is also better that that of other calculations using
composite methods of similar cost. The MUD of the new scheme on the barriers in the DBH24/08
subset (12 out of the 38 reactions in the other two sets) is 0.27 kcal mol-1
, better than that obtained
at the expensive CCSD(T,full)/aug-cc-pCV(T+d)Z level (0.46 kcal mol-1
) and comparable to the
2
most exact (and costly) Wn calculations (MUD=0.14 kcal mol-1
). The maximum unsigned
deviation (MaxUD) for all the reactions studied is 0.99 kcal/mol. G4 and M06-2X, on the other
side, exhibit MaxUDs of 6.7 and 8.0 kcal/mol respectively. The method was further tested against
a subset of the reactions in the databases, for which the geometry and energies of all species were
determined at the much more demanding CCSD(T)-F12//pVQZ-F12 level. These results showed
that Truhlar’s calculations in this subset are off the best values by a considerable amount, with an
rmse of 0.56 kcal/mol. As a consequence, a new dataset of barrier heights, SV20, is presented. The
SVECV-F12 procedure on this SV20 database results in rmse and MUD values of only 0.21 and
0.16 kcal/mol. The possible residual errors introduced by the approximations used for each
component of the method are tested against more sophisticated calculations and shown to be
accurate enough to obtain barriers well under the chemical precision limit at a reasonable cost for
molecules of interest in atmospheric chemistry.
Highly Accurate CCSDT(Q)/CBS Reaction Barrier Heights for a Diverse Set of Transition Structures: Basis Set Convergence and Cost-Effective Approaches for Estimating Post-CCSD(T) Contributions
