For the group 18 elements Kr, Xe, and Rn, where the atomic anion is unbound, the exponent of the additional diffuse functions was determined by extrapolation of the optimized exponents for elements in the same row (restricted to the p-block). For example, the Ga-Br. Visual inspection and curve fitting of the exponents across the relevant part of the p-block of the periodic table was used to determine if a linear, second-order polynomial, or exponential function was the most appropriate to use in determining the new exponent. The functional form used for each combination of group 18 element and basis set is detailed in the Supplementary Material, along with the freely optimized exponents / basis sets for the other heavy p-block elements.