A polarizable quantum mechanics and molecular mechanics model has been extended to account for the difference between the macroscopic electric field and the actual electric field felt by the solute molecule. This enables the calculation of effective microscopic properties which can be related to macroscopic susceptibilities directly comparable with experimental results. By seperating the discrete local field into two distinct contribution we define two different microscopic properties, the so-called solute and effective properties. The solute properties account for the pure solvent effects, i.e., effects even when the macroscopic electric field is zero, and the effective properties account for both the pure solvent effects and the effect from the induced dipoles in the solvent due to the macroscopic electric field. We present results for the linear and nonlinear polarizabilities of water and acetonitrile both in the gas phase and in the liquid phase. For all the properties we find that the pure solvent effect increases the properties whereas the induced electric field decreases the properties. Furthermore, we present results for the refractive index, third-harmonic generation ͑THG͒, and electric field induced second-harmonic generation ͑EFISH͒ for liquid water and acetonitrile. We find in general good agreement between the calculated and experimental results for the refractive index and the THG susceptibility. For the EFISH susceptibility, however, the difference between experiment and theory is larger since the orientational effect arising from the static electric field is not accurately described.

We report here the results for an ab initio approach to obtain the parameters needed for molecular simulations using a polarizable force field. These parameters consist of the atomic charges, polarizabilities, and radii. The former two are readily obtained using methods reported previously Swart, J Phys Chem A 1998, 102, 2399; Swart et al. J Comput Chem 2001, 22, 79), whereas here we report a new approach for obtaining atomic second-order radii (SOR), which is based on second-order atomic moments in scaled Voronoi cells. These parameters are obtained from quantumchemistry calculations on the monomers, and used without further adaptation directly for intermolecular interactions. The approach works very well as shown here for four dimers, where high-level coupled cluster with singles and doubles, and perturbative triples (CCSD(T)) and density functional theory (DFT) Swart-Solà-Bickelhaupt functional including Grimme's dispersion correction (SSB-D) reference data are available for comparison. The energy surfaces for the three methods are very similar, which is also the case for the interaction between a water molecule with either a chloride anion or a sodium cation. These latter systems had previously been used to criticize Thole's damped point-dipole method, but here we show that with the correct use of the method, it is perfectly able to describe the intermolecular interactions. This is most obvious for the induced dipole moment as function of the chloride-oxygen distance, where the direct (discrete) reaction field results are virtually indistinguishable from those obtained at CCSD(T)/aug-cc-pVTZ.