Interaction energy of Cl₂ and Br₂ with H₂ O: Exchange, dispersion and density the crucial ingredients

Modern Density Functional Theory models are now suitable for many molecular and condensed phase studies. The study of noncovalent interactions, a well-known drawback, is no longer an insurmountable obstacle through design and empirical corrections. However, using empirical corrections as in the DFT-D methods might not be an all-in-one solution. This work uses a simple system, X₂ -H₂ O with X = Cl or Br, with two different interactions, halogen-bonded (XB) and hydrogen-halogen (HX), to investigate the capability of current density functional approximations (DFA) in predicting interaction energies with eight different exchange-correlation functionals. SAPT(DFT) provides, for all the studied cases, better predictions than the widely used supermolecular approach. In addition, the components of the interaction energy suggest where some of the shortcomings originate in each DFA. The analysis of the functionals used confirms that PBE0 and ω-B97X-D have a physically correct behavior. Using SAPT(DFT) and PBE0, and ω-B97X-D, we obtained the interaction energy of Cl₂ and Br₂ inside different clathrate cages and satisfactorily compared with wavefunction results; hence, the lower and upper limits of this value are defined: Cl₂ @5¹² , -5.3 ± 0.3 kcal/mol; Cl₂ @5¹² 6² , -5.5 ± 0.1 kcal/mol; Br₂ @5¹² 6² , -7.6 ± 1.0 kcal/mol; Br₂ @5¹² 6³ , -10.6 ± 1.0 kcal/mol; Br₂ @5¹² 6⁴ , -10.9 ± 0.8 kcal/mol.