For PM3 the results are relatively insensitive to the type of atoms chosen with RMSE ú values of 4.4 to 4.7 kcal/mol (Table and Figure ). (Note that the RMSE value are computed using di erent numbers of charged species so the RMSE ú does not necessarily decrease when more parameters are optimized.) Similarly, the RMSE values computed using all the molecules in the MNSOL data set range from 2.9 to 3.2 kcal/mol as presented in Table . The cations are most sensitive to the choice of atom types with an RMSE range of 5.0 to 6.2 kcal/mol and the HCON-and HCONS-optimized radii set giving the best, and nearly identical, accuracy. For anions, the RMSE for the HCON radii (4.2 kcal/mol) is lower than that for HCONS (4.5 kcal/mol) and nearly identical to the lowest RMSE observed (4.1 kcal/mol). The HCON-optimized radii are therefore the best compromise.