Finding efficient methods for the quantum chemical treatment of protein structures: What are the effects of London-dispersion and basis-set incompleteness on peptide and water-cluster geometries?

We demonstrate how quantum chemical Hartree-Fock (HF) or density functional theory (DFT) optimisations with small basis sets of peptide and water cluster structures are decisively improved if London-dispersion effects, the basis-set-superposition error (BSSE) and other basis-set incompleteness errors are addressed properly. To achieve this better description, we concentrate on three empirical corrections to these problems recently advanced by Grimme and co-workers that lead to computational strategies that are both accurate and efficient. Our analysis encompasses a reoptimised version of Hobza’s P26 set of tripeptide structures, a new test set of conformers of cysteine dimers, and isomers of the water hexamers. These systems reflect features commonly found in protein crystal structures. In all cases, we recommend