More Dft Benchmarks – Sugars and “mindless” Test Sets

The first is an examination by Csonka and French of the ability of DFT to predict the relative energy of carbohydrate conformation energies. 1 They examined 15 conformers of αand β-D-allopyranose, fifteen conformations of 3,6-anydro-4-Omethyl-D-galactitol and four conformers of β-D-glucopyranose. The energies were referenced against those obtained at MP2/a-cc-pVTZ(-f)//B3LYP/6-31+G*. (This unusual basis set lacks the f functions on heavy atoms and d and diffuse functions on H.) Among the many comparisons and conclusions are the following: B3LYP is not the best functional for the sugars, in fact all other tested hybrid functional did better, with MO5-2X giving the best results. They suggest the MO5-2X/6311+G**//MO5-2x/6-31+G* is the preferred model for sugars, except for evaluating the difference between 1 C4 and 4 C1 conformers, where they opt for PBE/6-31+G**.