where σ i is a variable assigned to each of the N atoms of the NP that takes values {0, 1} depending on the atomic identity (i.e. platinum or gold in our study). E coor CNi corresponds to the contribution to the total energy of atom i with coordination number CN i , and E bond σiσj is the contribution of a bond formed between atoms i and j. The first term thus accounts for how often one of the two metals occupies positions with different coordination numbers, whereas the second accounts for the number of gold-gold, gold-platinum and platinum-platinum bonds of the system. We note that more negative values calculated from eq. 1 correspond to more stable homotops.