If c is selected too high, the algorithm quickly becomes unstable with corrections exceeding the scaling range. If this happens, the iteration should be interrupted and restarted by resetting the HREM scaling ladder to the original protocol given by Eq. 3 and by decreasing c. The choice of parameter ω depends on the selected EAF which in turn is a function of the communication overhead between replicas in a parallel execution. In the GROMACS program 31 patched with the PLUMED library, for example, the communication cost is high since configurations are exchanged among MPI processes. In ORAC, only the scaling factor are exchanged among replicas, with a minimal MPI overhead. For GROMACS, one should hence choose a smaller EAF (less frequent exchanges) and consequently a smaller ω by protracting the acceptance equalization scheme for a longer time until exchange equalization. Using ORAC, the equalization algorithm is faster since EAF and omega can be selected large. The equalization period (i.e. the time needed for the variance σ 2 of the exchange ratios to be below a given threshold) is a function of both c and ω.