Specifically, the toy model they are using is mentioned as an 'HP' (hydrophobic-polar) lattice model: https://en.wikipedia.org/wiki/Hydrophobic-polar_protein_f

Specifically, the toy model they are using is mentioned as an 'HP' (hydrophobic-polar) lattice model: https://en.wikipedia.org/wiki/Hydrophobic-polar_protein_folding_model https://en.wikipedia.org/wiki/Hydrophobic-polar_protein_fold... It's often drawn a little like a go board, with black and white circles on a grid. The intuition is that the model tries to capture the essence of a protein chain as a sequence of hydrophobic ('water-fearing') and polar (water 'soluble', in some sense) residues. The goal is to get a folded chain, with a hydrophobic core and a polar outside - roughly imitating the topology of a real protein. Obviously, you could just move the chain around at random, at each step calculating the energy of the conformation, and accept the move if it lowers the energy. This energy is calculated (I think) by giving one value to H-H neighbours, another to H-P neighbours, and another to H-H neighbours. At least, that's my understanding of HP models from a little interaction with the idea many years ago!