1. Assessing the i−TTM Model for Halide−Water Dimers:
Infrared Spectroscopy and Thermodynamics
Jiarong Zhou, Marc Riera, and Francesco Paesani
University of California, San Diego
Abstract and Background
MB-pol for H2O Interactions
i-TTM for Halide Interactions Infrared Spectroscopy
Halide ion hydration plays an important role in solvation mechanics, water−air
interfaces, and desalination processes. Properties of these systems are heavily
dependent on the interactions between the ionic species and the surrounding water
molecules, but experimental studies of these interactions can be difficult. In addition,
most ion−water computer models are represented in terms of empirical force fields. A
recent development of ab initio−based halide−water potential energy surfaces
(PESs) called ion−Thole-type model (i−TTM) can enable a better molecular
understanding of small X−(H2O)n clusters (X− = Cl−, Br−, I−). Here we assessed the
structural, spectroscopic, and thermodynamic properties of X−(H2O)2 clusters utilizing
the i−TTM potential. Infrared spectra and the dissociation enthalpy of the hydrogen
bond between the water dimer were investigated at different temperatures through
centroid molecular dynamics (CMD) and path integral molecular dynamics (PIMD)
simulations in the microcanonical and canonical ensembles.
Thermodynamics
The i-TTM potentials are derived from fits to electronic structure data, and include an
explicit treatment of the two-body repulsion, electrostatics, and dispersion
energies. Many body effects are represented through classical polarization within an
extended Thole-type model. By construction, the i-TTM potentials are compatible with
the flexible and fully ab initio MB-pol potential.
The results show that i-TTM can accurately
predict thermodynamic and spectroscopic
properties of halide−water systems in
different environments and conditions.
𝑉𝑡𝑜𝑡 = 𝑉𝑤
𝑖𝑛𝑡𝑟𝑎
+ 𝑉𝑤
𝑖𝑛𝑡𝑒𝑟
+ 𝑉 𝑖−𝑇𝑇𝑀
𝑉 𝑖−𝑇𝑇𝑀
= 𝑉 𝑇𝑇𝑀,𝑒𝑙𝑒𝑐
+ 𝑉 𝑇𝑇𝑀,𝑖𝑛𝑑
+
𝑖=1
𝑛
𝑉𝑖
𝑟𝑒𝑝
+ 𝑉𝑖
𝑑𝑖𝑠𝑝
𝑉𝑖
𝑑𝑖𝑠𝑝
=
𝑖=1
𝑛−1
𝑗=𝑖+1
𝑛
−𝑓 𝑅𝑖𝑗, 𝛿𝑖𝑗
𝐶6,𝑖𝑗
𝑅𝑖𝑗
6 𝑉𝑖
𝑟𝑒𝑝
=
𝑖=1
𝑛−1
𝑗=𝑖+1
𝑛
𝐴𝑖𝑗 𝑒 𝑏 𝑖𝑗 𝑅 𝑖𝑗
The energy of a system can be expressed as a many-body expansion of interaction
energies. For water clusters, this expansion converges very rapidly with just the 3-
body term, suggesting that the PES associated with a system containing N water
molecules can in principle be represented as a sum of low-order interactions.
EN-mer
= V1B
(i)
i=1
N
å + V 2B
(i, j)
i< j
N
å + V 3B
(i, j,k) +...+V NB
(1,...,N)
i< j<k
N
å
References
Acknowledgements
Center for Aerosol Impacts on
Climate and the Environment
http://paesanigroup.ucsd.edu/
The accuracy of the many-body polarizable
(MB-pol) model is retained from water dimers
to small clusters all the way to the condensed
phase as shown by the electronic structure
calculation comparisons to the CCSD(T) gold
standard values. The open symbols in panel
(a) are binding energies calculated using the
reference CCSD(T) dimer geometry.
Future Improvements
IHB1 IH FIHB2
A PIMD simulation in the NVT ensemble,
which represents constant number of
molecules, volume, and temperature,
was sampled through 100 different
trajectories of 100ps each. The
simulations ran at T = 10, 20, 40, 60, 80,
100, 120, 160, 200, 250, and 300K, and
the O−X−−O angle was monitored in real-
time. In addition, angular scans were
performed by calculating the minimum
energy of optimized halide−water
systems at the MP2/aug-cc-pVTZ level
as the O−X−−O angle was varied. The
first inflection point (d2E/dθ2 = 0) was
chosen as the cutoff angle that separates
the populations of the water dimer from
the independent water molecules. A van’t
Hoffs fit was used to quantify the
dissociation enthalpy and entropy of
each cluster.
ln 𝐾𝑒𝑞 = −
∆𝐻
𝑅𝑇
+
∆𝑆
𝑅
The results show that the dissociation
enthalpy of the water dimer is largely
reduced in the presence of the halide
ions. This reduction has been
characterized by a weakening of the
intermolecular interaction between the
water molecules due to vibrational
zero−point effects and a reduction in the
intrinsic binding energy.
Cluster ∆H (kcal/mol) ∆S (cal/mol*K)
Cl−(H2O)2 0.4756 1.6322
Br−(H2O)2 0.732 2.7198
I−(H2O)2 0.9183 3.0624
A CMD simulation in the NVE ensemble,
where E represents constant energy, was
sampled at similar conditions. A Fourier-
transform of the dipole-dipole time
correlation function was taken to generate
the infrared spectra. As the temperature
increased, the four distinct peaks that
represents each OH stretch collapses into
broad peaks due to the water molecules
becoming indistinguishable. Because of its
classical nature, the peaks were red-shifted
by around 300 wavenumbers as compared
to experimental data for I−(H2O)2 shown.
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This work is supported by CAICE, a NSF Center for Chemical Innovation (CHE-
1304527).
• Perform similar simulations and calculations of both thermodynamic and
spectroscopic properties of the F−(H2O)2 cluster
• Quantum molecular dynamics simulations in the NVT ensemble for better
thermodynamic properties
• Better sampling through replica exchange molecular dynamics (REMD)
• Quantum molecular dynamics simulations in the NVE ensemble for more accurate
infrared spectrum
• Physically meaningful definition of a cutoff angle to separate the populations of
hydrogen bonded water dimer from the independent water molecules when
determining the equilibrium constant