CHAPTER 7. Towards a Generalized Database of Atomic Polarizabilities

This chapter deals with the transferability of functional group polarizabilities. The motivation of this study is the need for a rapid evaluation of dielectric properties of materials consisting of aggregation of molecules (nanocrystals, fibres, films and crystals), with the purpose of rational design and efficient screening. Criteria based on the quantum theory of atoms in molecules define functional groups and provide a good transferability of their properties. This is true not only for properties depending solely on the ground state electron density, but also for those defined by mixtures of states, like the polarizability, which is the main subject of this study. In order to allow for the transferability, we use a generalized definition of a local coordinate system for each functional group, necessary to express the polarizability tensors in a fully self-contained way without biases. Transferability tests show that the best local coordinate system is based on the nuclear charge moment tensor of the functional groups. The a priori definition of the functional groups was further refined by clustering their polarizabilities, using a genetic algorithm according to some similarity criteria, which leads to an even better estimation of properties for molecules that were not used to build the original database. This procedure allows the rapid estimation of the polarizabilities of large molecules or, together with the dipole moment, to approximate the crystal field. Application of this approach concerns, among others, the calculation of the linear optical properties of crystals.