Considering N-methylacetamide (NMA) as a model compound, new interaction parameters are developed for the amide function in the GROMOS force field that are compatible with the recently derived 53A6OXY parameter set for oxygen-containing chemical functions. The resulting set, referred to as 53A6OXY+A, represents an improvement over earlier GROMOS force-field versions in the context of the pure-liquid properties of NMA, including the density, heat of vaporization, dielectric permittivity, self-diffusion constant and viscosity, as well as in terms of the Gibbs hydration free energy of this molecule. Assuming that NMA represents an adequate model compound for the backbone of peptides, 53A6OXY+A may be expected to also provide an improved description of polypeptide chains. As an initial test, simulations are reported for two beta-peptides characterized by very different folding properties in methanol. For these systems, earlier force-field versions provided good agreement with experimental NMR data, and the test shows that the improved description achieved in the context of NMA is not accompanied by any deterioration in the representation of the conformational properties of these peptides. (c) 2012 Wiley Periodicals, Inc.