Mass Scaling in MD Simulations. Besides gleaning insight into the dynamics of the system of interest, MD simulations are extensively employed to sample equilibrium configurational properties, i.e., quantities that depend on the particle positions, r, but not their momenta, p. When computing ensemble averages of these configurational properties, the classical partition function factorizes and, as a result, configuration space averages are independent of particle masses, ⟨A(r)⟩ = e -βE kin dp A(r)e -βEpot dr e -βE kin dp e -βEpot dr = A(r)e -βEpot dr e -βEpot dr (1) where A(r) is the configurational property of interest, E kin = p ⊤ M -1 p/2 and E pot are the total kinetic and potential energies, respectively, and β = 1/(k B T ), M is the mass tensor and ⊤ indicates the transpose. As a notable method that relies on eq 1, Metropolis Monte Carlo simulations of molecular systems accept or reject randomly proposed particle moves without referring to particle masses. Therefore, particle masses can be considered as adjustable parameters for the particular case of sampling equilibrium distributions of configurational properties. We note further that Newton's equation of motion, i.e., mass times acceleration equals force,