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The achievement of extensive and meaningful molecular dynamics simulations requires both the detailed knowledge of the basic features of the intermolecular interaction and the representation of the involved potential energy surface in a simple, natural and analytical form. This double request stimulated us to extend to ion-molecule systems a semiempirical method previously introduced for the description of weakly interacting atom-molecule aggregates and formulated in terms of atomic species-molecular bond interaction additivity. The method is here applied to the investigation of the prototypical M(+)-C6H6 systems (M = Li, Na, K, Rb and Cs) and some of its predictions are tested against accurate ab initio calculations. Such calculations have been performed by employing the MP2 method and large basis sets, privileging the description of the metal atoms. The agreement between potential energy scans semiempirically obtained and ab initio results is good for all the investigated geometries, thus showing that the adopted representation is in general able to reproduce all the main features of the potential energy surface for these systems. The role of the various noncovalent interaction components, as a function of the geometry and of the intermolecular distance in the M(+)-C6H6 complexes, is also investigated for a more detailed assessment of the results of the semiempirical method.

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The detailed knowledge of the basic aspects of molecular interactions and the representation of the involved potential energy surface in a proper analytical form are of paramount importance either to elucidate the nature of noncovalent interactions or to perform meaningful molecular dynamics simulations. To this aim, a recently developed semiempirical method, formulated in terms of atom/ion-molecular bond interactions, has been extended to investigate X(-)-C(6)H(6) systems (X = F, Cl, Br, I) and tested against highly correlated MP2 ab initio calculations. The role of the various components to the total interaction energy was also addressed by comparing the semiempirical contributions to their MP2 counterparts calculated using the symmetry adapted perturbation theory. The overall results, besides providing a more detailed picture of the interaction between anions and aromatic systems, pointed out that the current model is able to reproduce remarkably well the main features of the potential energy surface for the heavier X(-)-C(6)H(6) systems (X = Cl, Br, I), whereas for fluoride-benzene, the binding energies are underestimated as a consequence of the failure of the semiempirical method to describe the electrostatic interaction between a diffuse anion and a benzene at short range by means of a simple point charge model.