In most experiements and for most situations where NLO properties are used technically, there is a medium which surrounds the NLO active site or molecule. The effect of having such a medium is two-fold; it modifies the property of the single solute molecule (or system of molecules); and it modifies the external Maxwell field to a local counterpart. We have addressed these medium effects by developing reaction field and reaction field response theory in which the response equations of a given order is solved in the presence of a reaction field. We have also worked out simplifying models, like the so-called semiclassical model, for solvent induced properties. The reaction field and semi-classical models apply to the electronic NLO contributions, but can also be used in computations of the vibrational counterparts. The applicability is the same as for wave function computations of extended systems.
In current projects we are working on a molecular dynamics simulation technology for NLO effects in polar solvents.