CONSISTENT FORCE-FIELD STUDIES OF INTER-MOLECULAR FORCES IN HYDROGEN-BONDED CRYSTALS .2. BENCHMARK FOR THE OBJECTIVE COMPARISON OF ALTERNATIVE FORCE-FIELDS

A benchmark for the objective comparison of intermolecular force fields of carboxylic acids and amides is established. It includes all experimental data used by most authors of empirical energy studies hitherto. In order to demonstrate the utility of such a benchmark, a detailed comparison between three force fields is performed. First, root mean square deviations of various significant properties for the set of acid crystals and for that of amides are presented for all three force fields. This is followed by an individual comparison of these deviations in all the 14 carboxylic acids and the 12 amides of the benchmark. Many observations and conclusions of interest are derived from the benchmark comparison. Among them, the following are of a more general character : (1) The 9-6-1 CFF potential gives a best overall fit to experiment. (2) The calculated amide crystal properties fit generally better than those of carboxylic acid in all force fields, probably because the amide group forms twice as many hydrogen bonds, and therefore tends as a rule to form more extended networks of hydrogen bonds in the crystals. (3) There is a significant correlation of the degree of fit of most individual crystals in all force fields. The “bad” crystals are generally bad in all force fields, and similarly for the “good” ones. Possible reasons for these trends are discussed. © 1979, American Chemical Society. All rights reserved.