A fundamental aspect of the Direct Reaction Field (DRF) approach is the recognition of the polarizable nature of molecules. This property enables the molecules to "feel" the presence of other molecules through electrostatic interactions; furthermore, the molecules are able to respond to the presence of the other molecules through their (hyper)polarizabilities, which are well-defined physical properties of the molecules.

Thole devised a model to calculate molecular polarizabilities [3] from a set of atomic polarizabilities, which was reparameterized in 1998 [2]. This model enabled the development of a QM/MM approach where both the Quantumchemical and the Classical system could feel the presence of each other. This has successfully been applied to a range of systems: i.e. the active site of papain, solvachromatic shifts in acetone, dissociation reaction of tert-butyl chloride, "sudden" polarization in ethylene derivatives, many-body effects.

The QM/MM model is described in great detail in [12,14], while the completely classical part, including the gradient, is presented in [1].