Fig. 2.

Error distributions for dimer energies. (a) Distribution of energy differences between QMPFF results and the QM results at the MP2/6–311G** level for the interaction of 53 dimers in random orientations. The distributions for the training set (red, 1,881 conformations) and test set (blue, 4,220 conformations) are essentially identical, with rms energy errors of 0.39 and 0.42 kcal/mol, respectively. (b) The distribution of errors with QMPFF fitted to better quantum calculations at the MP2/aTZ(-hp) level are even closer to the QM results (0.27 kcal/mol rms error). The errors between the Merck force field MMFF94 and the QM results are larger (blue) with rms error of 0.51 kcal/mol. In b, the number of random dimer conformations is 5,093.