Accurate Thermochemistry for Large Molecules with Modern Density Functionals

The thermodynamic properties of molecules are of fundamental interest in chemistry and engineering. This chapter deals with developments made in the last few years in the search for accurate density functional theory-based quantum chemical electronic structure methods for this purpose. The typical target accuracy for reaction energies of larger systems in the condensed phase is realistically about 2 kcal/mol. This level is within reach of modern density functional approximations when combined with appropriate continuum solvation models and slightly modified thermostatistical corrections. Nine higher-level functionals of dispersion corrected hybrid, range-separated hybrid, and double-hybrid type were first tested on four common, mostly small molecule, thermochemical benchmark sets. These results are complemented by four large molecule reaction examples. In these systems with 70–200 atoms, long-range electron correlation is responsible for important parts of the interactions and dispersion-uncorrected functionals fail badly. When used together with properly polarized triple- or quadruple-zeta type AO basis sets, most of the investigated functionals provide accurate gas phase reaction energies close to the values estimated from experiment. The use of theoretical back-correction schemes for solvation and thermal effects, the impact of the self-interaction error for unsaturated systems, and the prospect of local coupled-cluster based reference energies as benchmarks are discussed.