A complete force field (MSXX) for simulation of all nylon polymers is derived from ab initio quantum calculations. Special emphasis is given to the accuracy of the hydrogen bond potential for the amide unit and the torsional potential between the peptide and alkane fragments. The MSXX force field was used to predict the structures, moduli, and detailed geometries of all nine nylons for which there are experimental crystal data plus one other. For nylon-(2n) with 2n less than or equal to 6, the alpha crystal structure (with all-trans CH2 chains nearly coplanar with the hydrogen bonding plane) is more stable, while for 2n > 6, gamma (with the alkane plane twisted by 70 degrees) is more stable. This change results from the increased importance of methylene packing interactions over H bonds for larger 2n. We find the highest Young’s modulus for nylon-7.