To determine the characteristic overpolarization of each method relative to gas phase experimental data, we fit a robust, nonparametric linear regression model (see Methods) to the dipole moments obtained using each method/basis set combination and the experimental gas phase dipole moments. The use of a robust method is warranted here to minimize the impacts of outliers, including cytosine, which in previous studies has been noted to have a potentially problematic experimental measurement. The point estimate provided by the Theil-Sen estimator that we used can be regarded as the median overpolarization of the method/basis set combination with respect to the experimental gas phase data.