where a, r, c are fitting parameters for specific DFT functional and basis set, c corresponds to limiting value of w at large ∆Edc/∆Eels ratio. In order to ensure physical meaning, w is set to 1 when it is greater than 1 in very rare cases. At B3LYP-D3(BJ)/aug-cc-pVTZ level, the fitted values are c = 0.472, a = 0.024 and r = 2.478, the corresponding map showing how w function varies with ∆Edc/∆Eels is shown in Fig. .