SOS Methods in Calculations of Electronic NLO Properties

Abstract

The sum-over-states technique which is extensively used in calculations of nonlinear optical properties, is presented and discussed. We focus on the electronic contributions to first- and second-order hyperpolarizability. The SOS approach to the calculation of the multiphoton absorption is also discussed. The various approximations to exact sum-over-states formulae are presented. In particular, we describe the so-called few-levels models, which are widely used in qualitative analysis of nonlinear electrical properties.