Apart from laboratory measurements carried out in ultra high vacuum (and the interstellar space) molecules don't appear as isolated species but in a chemical environment where the interaction with other molecules influences their properties, structure and reactivity. The interaction between molecules is of electric nature, governed by the simple Coulomb law, V(r) = Z₁ Z₂/(4πε₀ r), for the interaction between charged particles. But because of the many possible ways how the electric interaction can become apparent, e.g. as interaction between static electric (e.g. dipole) moments, or between static moments and (through the polarizabilities and hyperpolarizabilities) induced moments, or as van der Waals dispersion interaction the detailed description of intermolecular interactions in terms of molecular properties is usually rather complex. Together with the weakness of these interactions, this makes the determination, understanding and prediction of the potential energy surfaces and the structures and spectra of van der Waals complexes and the influence of intermolecular interactions on the chemical and physical properties and chemical reactions a rich and challenging field. Because of their electric nature intermolecular interactions are intimately connected with the electric or optical molecular properties and response theory.

Some problems which have been studied by us in collaboration with various partners are:

• Solvation effects on UV/Vis absorption spectra and two-photon spectra

• Prediction of the optical rotation of molecules in gas phase and solution by ab initio calculations

• Theoretical prediction of nonlinear optical properties of molecular crystals

• accurate ab initio calculations of van der Waals dispersion coefficients

• The description of intermolecular interactions between polyatomic molecules through distributed multipole moments, polarizabilities and van der Waals dispersion coefficients

• Pressure-dependence of linear and nonlinear optical properties of rare gases

Including solvent effects on the calculations of absorption and emission spectra and also geometrical structures in the ground and excited states in a computationally affordable manner is an important step towards a detailed understanding of photophysics and photochemistry of molecular systems in solution. In this regard, for the first time, the combination of Conductor-like Screening Model (COSMO) with correlated methods (CC2 and ADC(2)) beyond DFT, can provide powerful tools to take into account the electrostatic interactions between solute and polar environments in the simulation of the absorption (UV and MCD) and emission spectra and also excited state PES for small and medium sized molecules.