Issues with the 2 J(N, C α ) coupling Karplus parameters are discussed in Section S9 of the Supporting Information and elsewhere. The J coupling error for set 1 is very encouraging and is lower than both the OPLS-AA/M (1.16 ± 0.02) and AMOEBA force field errors (0.99). The errors for sets 2 and 3 with the excluded 2 J(N, C α ) term are similar in value.