Modifying def2-TZVPP: A Minimal Diffuse Addition for Reliable ΔE Versus ρ(BCP) Relationships in H-Bonded Systems

Basis set quality has a relevant role in the electronic structure description of hydrogen-bonded (H-bonded) systems, particularly for properties sensitive to the electron density in low-density intermolecular regions. Although the Ahlrichs def2-TZVPP basis set is widely used, it lacks the diffuse flexibility on hydrogen required for a reliable description of H-bonding regions. We introduce a minimal and physically motivated extension of def2-TZVPP consisting of a single p-type and a single d-type polarization function with diffuse exponents, added exclusively to hydrogen atoms, based on Density Functional Theory (DFT) calculations. The exponents of these functions were calibrated through a property-driven optimization using the ammonia-water complex and the electron density at the BCP as target property. The optimized basis set, denoted def2-TZVPP(pol-diff), maintains the correct ΔE_int versus ρ(BCP) behavior. To assess transferability, we validated the optimized basis set on a broad set of biologically relevant systems, including molecules exhibiting either inter and intramolecular H-bonds of varying strength and topology. The minimal diffuse augmentation preserves linear ΔE_int versus ρ(BCP) trends, improves the description of weak and medium-strength interactions, and yields consistently reliable electron densities in both inter- and intramolecular contexts. These results demonstrate that diffuse flexibility localized on hydrogen atoms is sufficient to overcome the limitations of smaller Ahlrichs basis sets near the BCP, providing an efficient and broadly applicable alternative for DFT studies of H-bonded systems.