  We propose a simple additive approach to simulate X-ray photoelectron spectra
(XPS) of macromolecules based on the $GW$ method. Single-shot $GW$ ($G_0W_0$)
is a promising technique to compute accurate core-electron binding energies
(BEs). However, its application to large molecules is still unfeasible. To
circumvent the computational cost of $G_0W_0$, we break the macromolecule into
tractable building blocks, such as isolated monomers, and sum up the
theoretical spectra of each component, weighted by their molar ratio. In this
work, we provide a first proof of concept by applying the method to four test
polymers and one copolymer, and show that it leads to an excellent agreement
with experiments. The method could be used to retrieve the composition of
unknown materials and study chemical reactions, by comparing the simulated
spectra with experimental ones.
