where α = ∆E MP2 c recovers the PW CBS MP2c energy by ensuring that n max is chosen large enough in order for Eq.38, respectively Eq.11, to be valid. For the systems studied, n max is between 10000 and 20000. To account for the sensitivity of the extrapolated value with respect to the fitting range, the results from all possible intervals ending at n max are calculated, and the final ∆E MP2 c value averaged among the intervals that respect Eq.38.