Function G(ω) is the VDOS of the disordered network. This equation works well for a-Si and a-Ge, "which are sp 3 bonded only, … The visible Raman spectra of disordered carbons are in marked contrast" (Ferrari and Robertson, and references therein). "In contrast, the Raman spectra of all disordered carbons are dominated by the relatively sharp G and D features of the sp 2 sites. This could be ascribed to the much greater cross-section of the π states. Nevertheless, the prevalence of G and D-like features, even in amorphous carbons with little graphitic ordering, requires explanation" (Ferrari and Robertson, and references therein).