Intricacies of Describing Weak Interactions involving Halogen Atoms within Density Functional Theory

In this work we assess the performance of different dispersion-corrected DFT approaches (M06, M06-2X, DFT-D3 and DCACP) in reproducing high-level wave function based benchmark calculations on the weakly bound halogen dimers (X2)2 and X2-Ar (for X=F,Cl,Br,I), as well as the prototype halogen bonded complexes H3CX-OCH2 (X=Cl,Br,I). In spite of the generally good performance of all tested methods for weakly bound systems, their performance for halogen-containing compounds varies largely. We find maximum errors in the energies with respect to the CCSD(T) reference values of 0.13 kcal/mol for DCACP, 0.22 kcal/mol for M06-2X, 0.47 kcal/mol for BLYP-D3 and 0.77 kcal/mol for M06, respectively. The root mean square deviations are 0.13 kcal/mol for DCACP and M06-2X, 0.44 kcal/mol for M06 and 0.51 kcal/mol for BLYP-D3.