The interaction of molecules with electromagnetic fields (homogenous or inhomogeneous, static or time-dependent) are related to a large variety of important molecular properties. The most well-known ones are the permanent dipole moment and the dipole polarizability, which describe the change of the energy in an homogenous electric field through first- and second-order in the field strength: E = f · μ + f² · α . In higher orders the interaction with electric fields is described by hyperpolarizabilities and — if magnetic fields are involved — magnetizabilities and hypermagnetizabilities, which are responsible for various nonlinear (magneto-) optical effects

• Kerr effect

• Pockels effect

• second and higher harmonic generation

• intensity-dependent refractive index

• Verdet effect

• Faraday effect

• Buckingham effect

• Cotton-Mouton effect

With the availability of strong lasers and magnets, the accurate knowledge of these nonlinear (magneto-) optical properties became essential for the understanding and the prediction of the behavior of molecules in strong fields. For some of these effects, however, accurate quantative measurements are difficult and/or only possible relative to a reference substance. Quantum chemical calculations are here of great help for a better understanding of these properties and for the validation of experimental results. Highly accurate ab initio calculations can serve for calibration of measurements. This has been the motivation for a number of investigations we have carried out in collaboration with international partners, in particular the Theoretical Chemistry Group at Århus University, Denmark:

• accurate calculation of frequency-dependent polarizabilities and hyperpolarizabilities

• calibration of ab initio methods for the calculation of static and frequency-dependent hyperpolarizabilities

• highly accurate calculations on the electric field induced second harmonic generation (ESHG) hyperpolarizability of the Ne atom

• validation and calibration of coupled-cluster response methods for the calculation of nonlinear magneto-optical properties

Many of these properties are related to intermolecular interactions (which are also of electric or electromagnetic nature) and there influence on molecular properties. E.g. the well-known van der Waals C₆ dispersion coefficients can be obtained from the frequency-dependent dipole polarizability.