A Hierarchy of Methods for the Energetically Accurate Modeling of Isomerism in Monosaccharides

The performance of different wave-function-based and density functional theory (DFT) methods was evaluated with respect to the prediction of relative energies for gas-phase monosaccharide isomers. A test set of 58 structures was employed, representing all forms of isomerism encountered in d-aldohexoses. The set was built from eight hexopyranose epimers by deriving subsets of isomers that include hydroxymethyl rotamers, anomers, ring conformers, furanose, and open-chain forms. Each subset of isomers spans a different energy range and involves various stereoelectronic effects. Reference energy values were obtained with coupled-cluster calculations extrapolated to the complete basis set limit, CCSD(T)/CBS. The tested CBS-extrapolated ab initio methods include various types of Møller-Plesset (MP) perturbation theory and the localized paired natural orbital coupled electron pair approach (LPNO-CEPA). Extensive benchmarking of DFT methods was carried out with 31 functionals. The results allow us to establish a hierarchy of methods that forms a reference guide for further computational studies. Among wave-function-based methods, LPNO-CEPA proved indistinguishable from CCSD(T), offering a promising alternative for a reference method that can be applied to larger systems. MP2 and SCS-MP2 follow closely, surpassing SOS-MP2 and MP3. The mPW2PLYP-D double hybrid and the Minnesota M06-2X hybrid meta-GGA are the best performing density functionals and are directly competitive with wave-function-based ab initio methods. Among the remaining functionals, B3PW91, TPSSh, mPW1PW91, and PBE0 yield the best results on average, while PBE is the best general-purpose GGA functional, surpassing meta-GGAs and several hybrids such as B3LYP. The choice of method strongly depends on the type of isomerism that needs to be considered, since many DFT methods perform well for purely conformational isomerism, but most of them fail to describe ring versus open-chain isomerism, where LYP-based GGA functionals perform particularly poorly.