Protein representations from all-atom to coarse-grained models. (A) All-atom. (B) OPEP. (C) Geometric representation of the protein intermediate-resolution model, PRIME, for polyalanine. Covalent bonds are shown with thick gray lines connecting united atoms for N–H, C=O, Cα-H, and CH3 side chain. At least one of each type of pseudobond is shown with a colored line. Pseudobonds are used to maintain backbone bond angles, consecutive Cα distances, and l-isomerization. The united atoms are not shown full size for ease of viewing. (D) UNRES. (E) Shea’s model.