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 ab initio 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 ab initio 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.