Because Equation ( ) is non-linear in its fitted parameters, a non-linear least-squares fitting is used to fit these parameters for every level of theory. To do this fitting, ten different randomly-generated initial values for the parameters are employed in each of 10 fits, and the best fit from these attempts is reported here. Each fit uses the least_squares function in scipy.optimize with the Trust Region Reflective method, 75 a 3-point method for calculating the Jacobian, and default numerical convergence criteria (e.g., reduce mean of errors).