Optimizing the basis set extrapolation parameter for weak interaction energy calculations using density functional theory

Accurate computation of weak interaction energies remains challenging due to basis set superposition error (BSSE) and the need for diffuse functions. While the counterpoise (CP) method is commonly used to correct BSSE, it increases computational cost and complexity. This work assesses the exponential-square-root extrapolation scheme for density functional theory (DFT) calculations of weak interactions. An optimized extrapolation exponent (α = 5.674) was obtained from a training set of 57 complexes and applied to a two-point extrapolation using def2-SVP and def2-TZVPP basis sets. The resulting interaction energies closely match those from CP-corrected ma-TZVPP calculations, with a mean relative error of ~2%. Although slightly less accurate than CP-corrected values, the extrapolation method requires only about half the computational time. Our results also show that diffuse functions are unnecessary for neutral systems when using CP with def2-TZVPP. Validation on the S66, L7, and NIAR20 benchmark sets and across other functionals confirms the method's robustness and transferability.