Physical accuracy in MM calculations requires maximally correct forces in addition to structures and energies. This requires inclusion of charge and polarizability fluxes in the energy function. Using our spectroscopically determined force field, which is designed to include these we present three examples of the improved physical predictions that result from the incorporation of charge fluxes: the reproduction of the water angle opening on going from the isolated molecule to the liquid, the reproduction of the psi peptide torsion potential with only a single threefold (almost zero barrier) Fourier term, and reproduction of the quantum-mechanical MD phi, psi map of a dipeptide analog. (c) 2006 Elsevier B.V. All rights reserved.