where d 2 x is the second moment of the distance to the center of the exchange hole, weighted toward asymptotic distances. (Details are provided in the Methods section.) The parameter C GDD is empirical and is determined so that ω GDD = ω IE for a data set of small molecules. Nevertheless, we find that GDD tuning performs well on large molecules also, and is often negligibly different from IE tuning in TD-DFT applications. Meanwhile, it reduces the cost and increases the convenience to determine an optimal range-separation parameter.