This assumption does not mean that such force field differences are necessarily large; indeed, small force field differences can result in large differences in optimized geometries . This is because many organic molecules have a large number of conformational minima often separated by relatively small barriers, so small force field differences may cause a molecule to optimize into different minima. Rather, we assume that force field differences which are large enough to substantially alter optimized geometries are of interest, even if the force field differences themselves are relatively small. All minimizations were performed with the same starting structure to ensure that differences observed are as attributable as possible to differences in force fields.