The multisite representation can be applied to residues with any number of titratable sites, including residues with only a single titratable site. In the latter case, two λcoordinates, corresponding to the protonated state (λ i,1 = 1, λ i,2 = 0) and deprotonated state (λ i,1 = 0, λ i,2 = 1) are introduced with a constraint on their sum (λ i,1 + λ i,2 = 1).