Local field factors interaction schemes

Proceeding from Are gas to the condensed phase many new issues appear. For NLO properties several additional complicaAons arise when an environment interacts with the system under investigation and external fields need to be considered. Already for the gas phase the proper definition of local field factors and the pressure dependence of (magneto-) optical properties is a difficult issue. In condensed phase, an important question is the proper definition of solute properties and the solvent effect for the electronic property itself. We refer the reader to a comprehensive discussion of solvent effects on NLO properties in a later Chapter. Some of the schemes for modeling solvent effects have been employed in connection with calculation of electronic NLO properties, also recenfiy at the CC level [212, 213]. This is still an area where much progress is expected in the coming years. 

Several methods have been developed in order to determine the macroscopic optical properties [63], of which the simplest is the oriented gas model due to Chemla et al. [64, 65] In that method, the hnear and nonlinear susceptibilities (Eq. (8.2) are calculated from simple tensor sums of the (hyper)polarizabihties of the molecules constituting the elementary unit cell. Corrective factors can subsequently be added to account for the effects of local electric fields. The relevance of this method is ensured provided the intermolecular interactions are weak, while the macroscopic responses are strongly dependent on the values of local field factors. More sophisticated schemes take into account the intermolecular interactions. They include the supermolecule model [66-69], where an aggregate of... 

As proposed by Munn and Hurst [74, 75], an elegant alternative is to employ the generalization of the electrostatic dipole interaction scheme proposed by SUber-stein for atoms [76, 77] to molecules, which consists in evaluating first the local field via the Lorentz-factor tensor [78] and then the macroscopic linear and NLO susceptibilities from the molecular responses calculated using quantum chemistry methods. To alleviate the limitations of the point dipole approximation, the molecule and its molecular responses are usually partitioned in submolecules [79]. Within this approach, the linear optical susceptibility tensor for a crystal with Z molecules labeled k (or Z submolecules labeled kj) per unit cell with a volume V reads ... 

---