Conformational hierarchies of weakly bonded systems: Accuracy of dispersion corrected DFT

It is well known that long-range dispersion interactions, important for stabilization of, e.g., molecular crystals, biomolecules and physisorption, are badly described by state-of-the-art \textit{xc} functionals in DFT, but naturally included in post-HF methods or empirically in force field simulations. We have implemented a semi-empirical $C_6/R^6$ correction [1,2] in the numeric atom-centered orbital based code FHI-aims [3] and obtained correction parameters by fitting to a database of high quality \textit{ab initio} calculations [2], improving on previous results due to the more accurate basis set (0.5 kcal/mol average error for binding energies using PBE+$C_6$). We assess the accuracy and impact of the correction on conformational energy \emph{hierarchies} of (i) (H$_2$O)$_n$ clusters ($n$=2-6), (ii) Ala$_2$ and Ala$_4$, and (iii) infinite polyalanine conformers, comparing to published post-HF results for (i) and (ii). Even though the relative energies are not changed for small H$_2$O clusters and Ala$_2$ compared to \textsc{DFT-GGA}, the impact of dispersion on the conformation hierarchy of larger systems is surprisingly large, reaching $\sim$1-4 kcal/mol for different polyalanine conformers. [1] S. Grimme, J. Comput. Chem. 25, 1463 (2004) [2] P. Jurecka et al., J. Comput. Chem. 28, 555 (2007) [3] V. Blum et al., FHI ab initio molecular simulations (FHI-aims) project.