Before discussing the differences between linear interpolating of the potential energy functions on the one hand, and of partial charges on the other hand, for computing the potential energy landscape of the titration coordinates, we briefly review the λ-dynamics approach that forms the basis for the constant pH molecular dynamics algorithm in GRO-MACS. λ-dynamics based constant pH MD simulations A titratable site i can exist in a protonated or deprotonated state. The protonation state affects the interactions between the site and the rest of the system. In constant pH MD simulations based on λ-dynamics, an additional coordinate λ i is introduced for each site i, and the potential energy function of the total system is continuously interpolated between the two protonation states along this coordinate i.e., V (λ i ). A fictitious mass m λ is assigned to each λ i -coordinate, and the coordinates evolve along with the Cartesian degrees of freedom of all atoms in the system, based on Newton's equations of motion. Thus, the total Hamiltonian of the system is