The Calculation of Intermolecular Potential Energy Surfaces

It is now possible to carry out accurate ab initio calculations on molecular complexes by a variety of techniques. The supermolecule approach is widely used, and is capable of high absolute accuracy, but it is subject to Basis Set Superposition Error, especially when electron correlation is taken into account, and this is a difficulty when accurate calculations of small interaction energies are required. Perturbation theory is not subject to BSSE, but perturbation methods as currently implemented are ‘uncoupled’; that is, the response of the electrons to the perturbation is not treated self-consistently. Nevertheless this method gives a more detailed description of the interaction than the supermolecule approach, and consequently provides more physical insight into the nature of the interaction. Both of these methods require calculations to be carried out at a wide range of dimer geometries if a full description of the potential energy surface is needed, and this is extremely time-consuming.