Accurate Modeling of Water Clusters with Density-Functional Theory Using Atom-Centered Potentials

The ability of atom-centered potentials (ACPs) to improve the modeling of water clusters using density-functional methods is explored. Water-specific ACPs were developed using accurate <i>ab initio</i> reference data to correct the deficiencies of the BHandHLYP density functional in the calculation of absolute and relative binding energies of water clusters. In conjunction with aug-cc-pVTZ basis sets and with or without dispersion corrections, it is possible to obtain absolute binding energies for water clusters containing up to 10 H₂O molecules to within 0.44 kcal/mol or 0.04 kcal/mol per water molecule. In contrast, dispersion-corrected BHandHLYP/aug-cc-pVTZ predicts binding energies with errors as large as 6 kcal/mol for (H₂O)₁₀ in the absence of ACPs. Therefore, the ACPs improve predicted binding energies in these clusters by more than an order of magnitude. The conformers of (H₂O)₁₆ and (H₂O)₁₇ were used to validate the application of ACPs to larger clusters. ACP-based approaches are able to predict the binding energies in (H₂O)_16,17 within a range of 0.3–2.2 kcal/mol (less than 1.3%) of recently revised <i>ab initio</i> wave function results. ACPs for basis sets smaller than aug-cc-pVTZ are also presented. However, the ability of the BHandHLYP/ACP approach to predict accurate binding energies deteriorates as the size of the basis sets decreases. Nevertheless, ACPs improve predicted binding energies by as much as a factor of 50 across the range of the basis sets studied. The BHandHLYP/aug-cc-pVTZ-ACP method was applied to (H₂O)₂₅ in order to identify the minimum-energy structure of a collection of proposed global minimum-energy structures. The BHandHLYP/aug-cc-pVTZ-ACP approach is an accurate and computationally affordable alternative to wave function theory methods for the prediction of the binding energies and energy ranking of water clusters