In truly disordered systems, such as molten polymer chains, any conformer is representative of the entire ensemble upon the assumption that conformation arises from the randomly joining of monomers. The spectroscopic responses of specific conformational features are then averaged out by considering a sufficient number of random conformations, and continuum solvent models can adequately represent the surrounding environment. Disordered peptides represent a peculiar case of 'conditionally disordered' systems, and carry features of both these two worlds. The prediction of experimental observables requires an accurate conformer representation to take into account the responses of each important structural feature. However, ab-initio methods are too costly to be applied to thousands of different conformers. Hence the necessity of rationally 'tidying up' the conformational phase space by means of clustering methods. Remarkably, once this is performed, the inclusion of more and more cluster central structures leads to a progressive convergence of the spectroscopic observables towards the ensemble average (Fig. ). This procedure is analogous to the rational expansion of wave-function basis sets in electronic structure calculation methods.