posted on 2021-03-12, 16:36authored byJoseph
P. Heindel, Sotiris S. Xantheas
We
present a detailed study of the many-body expansion (MBE) for
alkali metal and halide ion–water interactions and quantify
the effect of these ions on the strength of the surrounding aqueous
hydrogen bonding environment. Building on our previous work on neutral
water clusters [J. P. Heindel and S. S. Xantheas, J. Chem.
Theor. Comput.16 (11), 6843–6855 (2020)],
we carry out the MBE for the alkali metal and halide ion–water
clusters, Z+/–(H2O)9, where
Z = Li+, Na+, K+, Rb+,
Cs+, F–, Cl–, Br–, and I– and compare them with the
results for a pure water cluster with the same number of “bodies”,
viz., (H2O)10. The 2-B ion–water (I–W)
interaction accounts for a larger percentage of the total cluster
binding energy compared to a pure water cluster of the same size,
with the total 3-B term being smaller and of opposite sign (repulsive),
whereas higher order terms are essentially negligible. The same oscillating
behavior around zero for the MBE terms higher than the 5-B with a
basis set that was reported for water clusters is also observed for
the ion–water clusters considered here, with the basis set
superposition error (BSSE) corrections amending this as in the water
cluster case. A remarkable, linear anticorrelation between the total
2-B (I–W), the total 2-B (W–W), and also the 3-B (W–W–W)
interactions is found, quantifying the effect of the different ions
in disrupting and altering (weakening) the neighboring hydrogen bonded
water network: stronger (I–W) interactions result in weaker
(W–W) interactions. Additional linear correlations across the
two series of alkali metals and halide ions were found between the
3-B (I–W–W) and the 2-B (I–W) as well as between
the 3-B (I–W–W) and the 3-B (W–W–W) interactions,
suggesting the existence of previously unrealized underlying physics
governing these 2-B intermolecular and 3-B collective interactions.
Our results further suggest a universal behavior of the two different
families of ions (alkali metals and halides) for both the correlations
of the various components of the total binding energies and the estimate
of the 2-B BSSE correction, which is reported to follow a common profile
for ion–water and water–water interactions when cast
in terms of reduced distances and energies of the respective dimers.
We expect the current results that quantify the interplay between
ion–water and water–water interactions in aqueous clusters
to impact the development of classical, ab initio-based force fields for monatomic ion solvation, whereas the insights
into the nature of the BSSE to be critical in future ab initio-based, many-body molecular dynamics studies.