Fig. 4.

Training data set. The pK_a of the training data set compounds are used to derive a simple linear model that relates the free energy correction $\Delta G_{\text{corr}}^{\text{*}}$ to the experimental pK_a. Two linear models were derived: a global linear model (black dashed line), utilizing all data, and a piecewise linear model that applies to either neutral acids (subset QM1, blue) or to positively charged acids (subset QM2, green). a: Correlation between experimental and calculated pK_a of the training data set. The dashed line indicates ideal correlation with the gray band indicating 1 pK_a unit deviation. b: Global linear fit of the calculated $\Delta G_{\text{corr}}^{\text{*}}$ to the experimental pK_a. c: Linear fits of the calculated $\Delta G_{\text{corr}}^{\text{*}}$ to the experimental pK_a, split between the QM1 and the QM2 subsets. In (b) and (c) the dashed lines are linear models to the data, with shaded bands indicating 95% confidence intervals from 1000 bootstrap samples.