Modelling the dispersion energy for Van der Waals complexes

Strictly ab initio calculations of the dispersion energy are unfeasible in practice but for the smallest systems. A sensible alternative is to model the dispersion contribution through a damped multipolar expansion. This thesis proposes to represent the dispersion energy by means of a non-empirical, atom-atom model using damping functions scaled from 'exact' results for one electron-one electron systems. We start by investigating the scalability of ab initio calculated damping functions for closed-shell atom-atom dimers. Ab initio scaling parameters are employed to assess the quality of the damping functions yielded by a predictor scheme based on the charge overlap between the interacting monomers. The investigation of the scaling properties is extended to atom-linear molecule systems, focusing on the dependence on orientation of the short-range dispersion energy and how to account for it using isotropic damping parameters. We study the possibilities of an 'atomic' (multicentre) representation of the dispersion energy, in contrast to the conventional 'molecular' (single-centre) picture, devising a well-defined method to obtain 'atomic' dispersion coefficients from the computed molecular ones, as well as 'atomic' damping parameters. In all the studied cases, the 'atomic' approach describes more adequately the anisotropy of the interaction, through a localisation process of the charge overlap effects. The CO sub 2 /CO case, in particular, encourages to believe in the transferability of 'atomic' dispersion coefficients and damping parameters, which being confirmed by further work, the present results can be regarded as the basis of an universal and affordable model to estimate the dispersion contribution in intermolecular potentials