FIG. 1.

PMF profiles for $[{\text{NH}}_4^{+}\dots {\text{Cl}}^{-}]$ ion association using QM/MM-Ewald, QM/MM-Cutoff, QM/MM-IPS, and QM/MM-DSF methods with R_c = 12 Å. Similar to long-range methods such as QM/MM-Ewald and QM/MM-IPS, the QM/MM-DSF treatment eliminates the artificial free energy drift found in QM/MM-Cutoff. The PMF produced from the QM/MM-DSF simulations is in close agreement with that from the cutoff-based long-range method QM/MM-IPS.