Mattioli et al. investigated the uracil cluster interactions using XPS experimental and ab-initio studies. Uracil hexamer considered in this study is symmetric, with the upper and lower layers consisting of identical uracil molecules. The core-ionization values for each individual uracil O(1s) are outlined in Table . The structure of this hexamer is reported in the supplementary information. The uracil hexamer at CC-PCVTZ had 2400 basis functions;