Stereoelectronic substituent effects in saturated main group molecules: Severe problems of current Kohn-Sham density functional theory

The Hartree-Fock method, two common density functionals (PBE and B3LYP), and two new functionals (B97-D and B2PLYP) together with very large AO basis sets are used to compute the isomerization energies for substituted (RH, F, Cl) branched to linear alkanes and silanes. The results of accurate SCS-MP2 computations are taken as reference. These reactions are an important test of how nonlocal electron correlation effects on medium-range lengths scales in saturated molecules are treated by approximate quantum chemical methods. It is found that the unacceptably large errors observed previously for hydrocarbons persist also for the here considered more polar systems. Although the B97-D and B2PLYP functionals provide improved energetics, the problem is not fully solved, and thus these systems are suggested as mandatory benchmarks for future density functionals.