We present a theoretical characterization of the interaction of Cl and Br in the 5 and 56 clathrate cages, respectively, based on energy partitioning analysis and a study of the electronic shifts associated with transitions to the main valence bands. Our analysis clearly shows that while Br@56 does not show halogen bonding interactions in its equilibrium geometry, Cl@5 presents all the characteristics expected for halogen bonding. This is accomplished by the interaction of the usual sigma-hole with the lone pair of the closest oxygen atom involved in hydrogen bonding within the cage framework, though breaking of the hydrogen bond is not required. This possibility, which had not been considered in previous analyses, opens up a new way of looking at the interactions of dihalogens with the nearest water molecules in the cage.

The guest-host intermolecular potentials for the valence excited states of Br in the tetrakaidecahedral(T) and pentakaidecahedral(P) clathrate cages have been calculated using ab initio local correlation methods. We find that the excited states are more strongly bound than the corresponding ground states even in the small T cage where bromine has a tight fit. The angular dependence of the interaction energies is quite anisotropic; this reflects in the corresponding electronic shifts where regions of maxima for blue-shifts in the T cage indicate the presence of halogen bonding. We predict a large temperature dependence of the electronic shifts and compare absolute values with recent experimental studies. This stringent test indicates the reliability of local correlation treatments to describe weak intermolecular forces in ground and excited states.

The guest-host intermolecular potentials for the ground states of Br in the tetrakaidecahedral (T), pentakaidecahedral (P), and hexakaidecahedral clathrate (H) cages have been calculated using ab initio local correlation methods. Applying the local correlation energy partitioning analysis together with first-order symmetry adapted perturbation theory, we obtain a detailed understanding of the nature of the interactions. In particular, the debated question concerning the possible presence of halogen bonding (XB) is carefully analyzed. In the case of the T cage, given its smaller size, the Br-O distance is too short leading to a larger exchange-repulsion for XB orientations which therefore do not represent minima. For the other two cages, the Br-O distance is too large leading to little orbital overlap effects and thus weaker donor-acceptor interactions; however, these orientations coincide with the global minima.

The performance of local correlation methods is examined for the interactions present in clusters of bromine with water where the combined effect of hydrogen bonding (HB), halogen bonding (XB), and hydrogen-halogen (HX) interactions lead to many interesting properties. Local methods reproduce all the subtleties involved such as many-body effects and dispersion contributions provided that specific methodological steps are followed. Additionally, they predict optimized geometries that are nearly free of basis set superposition error that lead to improved estimates of spectroscopic properties. Taking advantage of the local correlation energy partitioning scheme, we compare the different interaction environments present in small clusters and those inside the 5(12)6(2) clathrate cage. This analysis allows a clear identification of the reasons supporting the use of local methods for large systems where non-covalent interactions play a key role.