SVECV-f12: benchmark of a composite scheme for accurate and cost effective evaluation of reaction barriers.

Abstract

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.

Energetics of the OH radical H-abstraction reactions from simple aldehydes and their geminal diol forms

“…The other two DFT methods, M06-2X-D3 and PW6B95-D3, are over this threshold on more occasions. This shows that, while the DFT methods here used to obtain the equilibrium geometry of the stable bound species on top of which the other calculations are performed are equivalent (provided there are no multi-conformation or degeneracy problems, as we showed in previous publications [44,63]) not all of them are equally useful to obtain relative energies, unless further corrections are carried on. The size of the average error of the three DFT procedures, B2PLYP (3.0 kJ mol -1 ), M06-2X-D3 (3.5 kJ mol -1 ), and PW6B95-D3 (7.5 kJ mol -1 ), shows that these methods are reasonably appropriate for this type of study on larger molecules for which the coupled-cluster calculations cannot be performed, although B2PLYP should be the method of choice, resources allowing.…”

“…The simple CBS-QB3 [39,40] composite protocol, which relies on the B3LYP [41] DFT method to calculate optimum geometries and CCSD(T) [29], MP4SDQ [42], and MP2 [43] calculations with different basis sets to calculate the energies, was one of the procedures employed for the fast determination of all the species. Two more sophisticated wavefunctionbased composite methods were also employed, namely SVECV-f12 [44][45][46] and jun-Cheap (jChS) [47][48][49], which in both cases imply the calculation of optimum geometries at the DFT level, calculation of the complete basis set (CBS) [50] energy on this fixed geometry by CCSD(T)-F12 [51] methods in the first case, and a combination of MP2 and CCSD(T) in the second case, and adding core-valence correlation energy employing MP2 calculations using the cc-pwCVTZ basis set [52]. A simulated bulk water environment was introduced in the calculation employing the self-consistent reaction field developed by Tomasi and coworkers, called Polarizable Continuum Model (PCM) [53].…”