We computed the 𝐹 ! atomic , 𝐹 ! solvent , and 𝐹 ! mixed terms of equation ( ) for all the amino acids, as well as for nucleic acid bases and backbones, for both SAXS and SANS. Concerning proteins, the three terms per amino acid were calculated and averaged over 1,000 frames extracted at equidistant intervals from a 2.7 µs MD trajectory of GSN. The heterogeneous structural composition of this 755-residue protein makes it an ideal model for a comprehensive conformational sampling. Indeed, in addition to encompassing all the standard amino acids, GSN features an IDP-like N-terminal region of ~25 residues and six structured domains rich in alpha- behaviour of the histidine with both the δ-and ε-nitrogen of the imidazole ring protonated. A different strategy was employed for nucleotides. Considering the lower accuracy of the FF for nucleic acids 55 , we preferred to calculate and average the terms from non-redundant molecular structures obtained from the PDB. We used a set of 167 non-complexed DNA structures and a set of 75 RNA structures 57 that we had already prepared and used in our previous work . To validate the parameters, 120 DNA and 43 RNA structures with no missing heavy atoms were selected from these repositories as the training subset, while the remaining structures were used as the validation subset. The final terms were computed on the two complete repositories (Table ). We derived the form factors components of the five nucleobases (adenine, cytosine, thymine, uracil, and guanine), the phosphate group, and the DNA and RNA pentose sugars. In addition, we included two other DNA/RNA bead types for the 5'-end and the 3'-end pentose sugar with hydroxyl moiety at carbon C5' and C3', respectively. Finally, each term belonging to either protein or nucleic acid was fitted to a sixth-order polynomial, this means that 𝐹 ! atomic , 𝐹 ! solvent , and 𝐹 ! mixed are described by a total of 21 parameters.