To ascertain the effective scaling of cost versus system size we will present timing data for homologous systems starting with Cl -(H 2 O) n clusters of increasing size, treating (H 2 O) n as a single monomer. Figure presents timing data for XSAPT + MBD calculations. The quantity that is plotted is the total CPU time aggregated across all 20 processors that were used, not the time-to-completion. The latter is much shorter, as may be inferred from the reasonably good parallel efficiencies that are documented in Table , but it is total CPU no. water molecules no. water molecules t = 0.00267n t = 0.02558n that captures the true cost of the calculation. 189 Fits of this total time t to the function t = αn β reveal that the scaling is effectively cubic, for both the XPol-SCF procedure (which contributes 80% of the total cost, for the largest systems examined) and for the post-SCF SAPT and MBD corrections (comprising the remaining 20% of the cost).