The classical f-sum rule is generalized using quantum mechanical Thomas–Reiche–Kuhn sum rule to include nucleonic contribution, i.e. lattice vibrations. The sum rule is formulated for the transition strength function defined as a continuous condensed-matter equivalent of the oscillator strength known for discrete transitions in atomic spectra. The application of such formulated sum rule allows construction of dispersion models containing a fitting parameter directly related to the atomic density of material. The dielectric response expressed using the transition strength function is split into individual contributions such as direct and indirect electronic interband transitions including excitonic effect, excitations of electrons to the high-energy states existing above the conduction band, core-electron excitations and phonon absorption. The presented models reflect understanding of structure of disordered and crystalline materials on the basis of quantum theory of solids. The usual term ‘joint density of states’, that should be used only for electronic transitions in the one-electron approximation, is replaced by the more general term ‘transition density’.