To understand the electronic structure of CuBiSeCl2 in more detail, experimental measurement of the valence band using XPS was combined with partial Density of States (pDoS) calculations. Figure (a) shows valence band XPS compared to pDoS that has been corrected to account for realistic photoemission processes including lifetime broadening and spectrometer resolution. The broadening processes used are discussed in greater detail in the Supplementary Information (S25-26).