Reliable Structures and Electronic Energies of Small Water Clusters Using Density Functional and Local Correlation Coupled Cluster Model Chemistries

In this paper we have assessed the ability of the domain-based
local pair natural orbital (DLPNO)-CCSD(T) method to match the explicitly
correlated CCSD(T) relative energies of (H2O)n=3–7 isomers along with the impact of the
level of theory utilized to optimize the water cluster geometries.
The benchmark structures were optimized using a 2-body:Many-body procedure
in which all of the 1- and 2-body contributions are computed using
CCSD(T) while all of the higher-order many-body interactions are computed
using MP2 (denoted CCSD(T):MP2). Benchmark relative energies were
computed for these CCSD(T):MP2 optimized geometries with explicitly
correlated CCSD(T)-F12b single point energies (SPEs) using the cc-pVQZ-F12
and cc-pV5Z-F12 basis sets augmented with diffuse functions on the
O atoms. The benchmark structures and energies were used to gauge
the performance of less demanding computational protocols. For example,
DLPNO–CCSD(T) computations on the 31 benchmark structures with
the analogous family of correlation consistent basis sets (cc-pVNZ
for H and aug-cc-pVNZ for O, or simply haNZ where N = D-6) were used
to estimate relative energies at the complete basis set (CBS) limit
via three-point extrapolations. When compared to the CCSD(T)-F12 benchmark
data, the mean absolute differences (MADs) were ≤ 0.13 kcal/mol
when triple-ζ and larger basis sets were employed. Using these
DLPNO–CCSD(T) results, we demonstrate that 2 less-demanding
geometry optimization procedures, specifically the ωB97X-D density
functional theory (DFT) method paired with the 6–31++G(d,p)
basis set and the density-fitted MP2 method paired with the haTZ basis
set, give structures that yield nearly identical relative energies
(MADs of only 0.07 and 0.02 kcal/mol, respectively, when comparing
DLPNO–CCSD(T)/ha6Z data). In addition, we show how the presence
or absence of diffuse functions in the basis sets used for DLPNO–CCSD(T)
SPEs impact the quality of the relative energies. The protocol that
combines ωB97X-D/6–31++G(d,p) optimized structures with
DLPNO–CCSD(T) SPEs using triple-ζ or higher Dunning basis
sets that include augmentation with diffuse functions on the oxygen
atoms provides a fast and accurate method for determining the relative
electronic energies of (H2O)n=3–7 water cluster isomers.