Extrapolation and Scaling of the DFT-SAPT Interaction Energies toward the Basis Set Limit.

The dispersion energy term in the symmetry-adapted perturbation theory based on the density functional theory (DFT-SAPT) converges rather slowly with basis set size. Accurate results for large complexes, where only calculations in small basis sets are practical, can be obtained by extrapolation to the complete basis set limit (CBS). In this paper, we propose an extrapolation scheme with the variable exponent optimized specifically for the DFT-SAPT calculations in correlation-consistent basis sets with diffuse functions. Another way to improve the accuracy term at no additional cost is to scale the dispersion term by a fixed amount. We present the scaling factors averaged over a balanced set of 10 model complexes. The results of these schemes are compared to the high-quality DFT-SAPT/CBS interaction energies in small complexes obtained by fitting to a series of basis sets up to aug-cc-pV5Z and to the CCSD(T)/CBS interaction energies. It is shown that even the cheapest extrapolation scheme yields results that are limited by the accuracy of the DFT-SAPT approach rather than by the basis set convergence. Scaling the dispersion term allows accurate interaction energies as well as their components to be obtained using just the aug-cc-pVDZ basis set; such a calculation can be applied to complexes consisting of up to 50 first-row atoms.