where q MM is a vector of size N B containing all MM point charges, q 0 MM is the vector of the undistorted MM point charges as provided by the employed force field, and ∆q MM is a collective variation of these point charges, which depends on a parameter ∆q that controls the size of the variation. We chose the collective variations of the MM point charges such that I ∆q MM,I = 0, i.e., the sum of the MM point charges is preserved.