© Paesani Research Group. All rights reserved.
65. Development of a “first principles" water potential with flexible monomers: Dimer potential energy surface,
VRT spectrum, and second virial coefficient, V. Babin, C. Leforestier, F. Paesani, J. Chem. Theory Comput. 9, 5395
The development of a “first principles” water potential with flexible monomers (MB-pol) for molecular simulations
of water systems from gas to condensed phases is described. MB-pol is built upon the many-body expansion of
the intermolecular interactions, and the specific focus of this study is on the two-body term (V2B) representing the
full-dimensional intermolecular part of the water dimer potential energy surface. V2B is constructed by fitting
40,000 dimer energies calculated at the CCSD(T)/CBS level of theory and imposing the correct asymptotic
behavior at long-range as predicted from “first principles”. The comparison of the calculated vibration–rotation
tunneling (VRT) spectrum and second virial coefficient with the corresponding experimental results demonstrates
the accuracy of the MB-pol dimer potential energy surface.
64. Many-body convergence of the electrostatic properties of water. G.R. Medders, F. Paesani, J. Chem. Theory
Comput. 9, 4844 (2013).
The many-body convergence of the dipole moment and the dipole–dipole polarizability of water is
investigated. It is found that, for systems of low symmetry like the water clusters examined here,
simple measures such as dipole magnitudes and average polarizabilities may lead to an incomplete
interpretation of the underlying physics. Alternative metrics are introduced that allow for an
unambiguous characterization of both properties. The convergence of the many-body decomposition
of the total dipole and the polarizability is studied for (H2O)N, with N = 4 - 6 being minimum energy
water clusters and N = 14 being clusters that were extracted from condensed phase simulations.
For these clusters, it is demonstrated that both the total dipole and polarizability are almost entirely
pairwise additive, with three-body terms contributing less than 4% and all higher-order terms being
63. Molecular mechanisms of water-mediated proton transport in MIL-53 metal-organic frameworks. F. Paesani,
J. Phys. Chem. C 117, 19508 (2013).
Metal–organic frameworks have recently been proposed as promising proton conducting materials for
application in fuel cell technologies. Here, molecular dynamics simulations are used to reveal the microscopic
mechanisms associated with water-mediated proton transport in the MIL-53 materials as a function of temperature,
water loading, and pore size. The structure of the hydrated proton is found to resemble that of a distorted Zundel
complex when the framework closes into a narrow-pore configuration. A transition to Eigen-like structures is then
observed at higher water loading when the pores open as a result of the breathing effect. Although the free-energy
barriers to proton transfer at room temperature are lower than in bulk water, proton transport in MIL-53 is largely
suppressed, which is attributed to the low water mobility inside the pores. Faster proton diffusion is found at
higher temperature, in agreement with conductivity measurements.
62. The curious case of the water hexamer: Cage vs. Prism. V. Babin, F. Paesani, Chem. Phys. Lett. 580, 1 (2013).
Small water clusters, such as the hexamer, provide a unique opportunity to advance the molecular-level
understanding of water in all its phases. In particular, the water hexamer is the smallest cluster that possesses
several nearly iso-energetic non-planar isomeric forms whose relative stability at low temperatures can be
probed experimentally and investigated theoretically. Here, we report on the equilibrium populations of the
isomers in the temperature range from 30 K to 150 K for both H2O and D2O as predicted by four different
water potentials. The simulations, performed using path-integral molecular dynamics combined with the
replica exchange method, highlight some deficiencies of empirical water models while providing support
for the accuracy of more recent ab initio-based potentials. The theoretical predictions for the cage/prism
isomeric equilibrium upon isotopic substitution suggest that rotational spectra measured for the deuterated
cluster could deliver further insights on the ground-state properties of the water hexamer.
61. Systematic study of structural and thermodynamic properties of HCl(H2O)n clusters from semiempirical replica
exchange simulations, W. Lin, F. Paesani, J. Phys. Chem. A 117, 7131 (2013).
The structural and thermodynamic properties of HCl(H2O)n clusters with n = 4 - 10 are studied using
Born-Oppenheimer replica exchange molecular dynamics simulations with the PM3-MAIS semiempirical
Hamiltonian. Independently of the cluster size, the simulations predict that HCl exists in the dissociated form
in all low-energy isomers. Different local structures are identified within the clusters due to the presence of the
dissociated proton, including Zundel, Eigen, Eigen-like, H7O3+, and intermediate Zundel - Eigen configurations.
As the cluster size increases, several groups of isomers are identified, whose relative stabilities vary as a function
of temperature. A detailed analysis of the heat capacity indicates that the melting behavior of HCl(H2O)n clusters
is strongly size-dependent. In particular, melting is observed in clusters with n = 7 - 10 in the temperature range
T = 100 - 150 K. By contrast, melting is not observed in clusters with n = 4 - 6.
60. Chemically cross-linked metal-organic frameworks. C.A. Allen, J.A. Boissonnault, J. Cirera, R. Gulland, F. Paesani,
S.M. Cohen, Chem. Comm. 49, 3200 (2013).
In this study, we report the synthesis of canonical isoreticular metal–organic frameworks (IRMOFs) containing
interligand crosslinks. Chemically crosslinking two molecules of 2-amino-1,4-benzene dicarboxylic acid (NH2-BDC)
gives ligands that readily form IRMOF-1 analogs, producing crosslinked MOFs that may be designed to have
novel properties. In general, the properties of these MOFs are similar to the parent IRMOF-3; however, a notable
difference in guest entrapment was observed with IRMOF-3-AM4XL. The robust inclusion of DMF in this MOF
suggests that the physical properties of the MOF can be substantially altered by even very small changes in
tether length. This observation, combined with the ability to incorporate additional chemical functionality into
the tethering group, opens up a new subclass of MOFs with even more highly tailored chemical and physical
properties solely within the pores.
59. Negative ion photoelectron spectroscopy reveals thermodynamic advantage of organic acids in facilitating
formation of bisulfate ion clusters: Atmospheric implications. G.-L. Hou, W. Lin, S. H. M. Deng, J. Zhang,
W.-J. Zheng, F. Paesani, X.-B. Wang, J. Phys. Chem. Lett. 4, 779 (2013).
Recent lab and field measurements have indicated critical roles of organic acids in enhancing new atmospheric
aerosol formation. We report a combined negative ion photoelectron spectroscopic and theoretical investigation
of molecular clusters formed by HSO4– with succinic acid (SUA), HSO4–(SUA)n along with HSO4–(H2O)n and
HSO4–(H2SO4)n. It is found that one SUA molecule can stabilize HSO4– by ~39 kcal/mol, three times the
corresponding value that one water molecule is capable of (~13 kcal/mol). Molecular dynamics simulations and
quantum chemical calculations reveal the most plausible structures of these clusters and attribute the stability of
these clusters to the formation of strong hydrogen bonds. This work provides direct experimental evidence
showing significant thermodynamic advantage by involving organic acid molecules to promote formation and
growth in bisulfate clusters and aerosols.
58. A critical assessment of two-body and three-body interactions in water. G.R. Medders, V. Babin, F. Paesani,
J. Chem. Theory Comput. 9, 1103 (2013).
The microscopic behavior of water under different conditions and in different environments
remains the subject of intense debate. A great number of the controversies arise due to the
contradictory predictions obtained within different theoretical models. Relative to conclusions
derived from force fields or density functional theory, there is comparably less room to dispute
highly correlated electronic structure calculations. In this study, a detailed analysis of the two-
and three-body water interactions evaluated at the CCSD(T) level is carried out to quantitatively
assess the accuracy of several force fields, DFT models, and ab initio based interaction potentials
that are commonly used in molecular simulations. On the basis of this analysis, a new model,
HBB2-pol, is introduced which is capable of accurately mapping CCSD(T) results for water dimers
and trimers into an efficient analytical function. The accuracy of HBB2-pol is further established
through comparison with the experimentally determined second and third virial coefficients.