67. Fast and slow proton transfer in ice: The role of the quasi-liquid layer and hydrogen-bond network. K. Park,
W. Lin, F. Paesani, J. Phys. Chem. B 118, 8081 (2014). [link]
The proton mobility in ice is studied through molecular dynamics simulations carried out with
a newly developed ab initio-based reactive force field, aMS-EVB3/ice. The analysis of both
structural and dynamical properties of protonated ice as a function of temperature indicates
that the mobility of excess protons at the surface is largely suppressed, with protons becoming
essentially immobile at temperatures below 200 K. In contrast, fast proton transfer/transport
can exist in bulk ice Ih at low temperature through connected regions of the proton-disordered
hydrogen-bond network. Based on the simulation results, it is shown that the mechanisms
associated with proton transfer/transport in both bulk and interfacial regions of ice are largely
dependent on the local hydrogen-bond structure surrounding the charge defect. A molecular-level
picture of the mechanisms responsible for proton transfer/transport in ice is then developed and
used to interpret the available experimental data.
69. Water dynamics in metal-organic frameworks: Effects of heterogeneous confinement predicted by
computational spectroscopy, G.R. Medders, F. Paesani, J. Phys. Chem. Lett. 5, 2897 (2014). [link]
The behavior of water confined in MIL-53(Cr), a flexible metal–organic framework (MOF), is
investigated through computational infrared spectroscopy. As the number of molecules adsorbed
inside of the pores increases, the water OH stretch band of the linear infrared spectrum grows in
intensity and approaches that of bulk water. To assess whether the water confined in MIL-53(Cr)
becomes liquid-like, two-dimensional infrared spectra (2DIR) are also calculated. Confinement
effects result in distinct chemical environments that appear as specific features in the 2DIR spectra.
The evolution of the 2DIR line shape as a function of waiting time is well described in terms of the
orientational dynamics of the water molecules, with chemical exchange cross peaks appearing at a
time scale similar to the hydrogen bond rearrangement lifetime. The confining environment
considerably slows the hydrogen bond dynamics relative to bulk as a result of the competition
between water–framework and water–water interactions.
70. Theoretical modeling of spin crossover in metal-organic frameworks: [Fe(pz)Pt(CN)4] as a case study. J. Cirera,
V. Babin, F. Paesani, Inorg. Chem. 53, 11020 (2014). [link]
MOFs with spin-crossover behavior are promising materials for applications in memory storage
and sensing devices. A key parameter that characterizes these materials is the transition
temperature T1/2 between the low-spin and high-spin species. In this study, we describe the
development, implementation, and application of a novel hybrid Monte Carlo / molecular dynamics
method that builds upon the Ligand Field Molecular Mechanics approach and enables the modeling
of spin-crossover properties in bulk materials. The new methodology is applied to the study of a
spin-crossover MOF with molecular formula [Fe(pz)Pt(CN)4] (pz = pyrazine). The T1/2 value,
calculated from the temperature dependence of the magnetization curve, is in good agreement with
the available experimental data. The comparison between the spin-crossover properties of the
isolated secondary building block of the framework and the bulk material reveals the origin of the
different behavior of the two systems.
© Paesani Research Group. All rights reserved.
71. Communication: On the consistency of approximate quantum dynamics simulation methods for vibrational
spectra in the condensed phase. M. Rossi, H. Liu, F. Paesani, J.M. Bowman, M. Ceriotti, J. Chem. Phys. 141,
181101 (2014). [link]
Here, we perform a systematic comparison between these two philosophies for the description of quantum effects
in vibrational spectroscopy, taking the Embedded Local Monomer model and a mixed quantum-classical model as
representatives of the first family of methods, and centroid molecular dynamics and thermostatted ring polymer
molecular dynamics as examples of the latter. With few exceptions, the different techniques yield IR absorption
frequencies that are consistent with one another within a few tens of cm−1. Comparison with classical molecular
dynamics demonstrates the importance of nuclear quantum effects up to the highest temperature, and a detailed
discussion of the discrepancies between the various methods let us draw some (circumstantial) conclusions
about the impact of the very different approximations that underlie them.
68. Development of a “first principles" water potential with flexible monomers. III: Liquid phase properties.
G.R. Medders, V. Babin, F. Paesani, J. Chem. Theory Comput. 10, 2906 (2014). [link]
The MB-pol full-dimensional water potential introduced in the first two papers of this series
[J. Chem. Theory Comput. 2013, 9, 5395 and J. Chem. Theory Comput. 2014, 10, 1599] is employed here
in classical and quantum simulations of liquid water under ambient conditions. Comparisons with the available
experimental data for several structural, thermodynamic, and dynamical properties indicate that MB-pol
provides a highly accurate description of the liquid phase. Combined with previous analyses of the dimer
vibration–rotation tunneling spectrum, second and third virial coefficients, and cluster structures and energies,
the present results demonstrate that MB-pol represents a major step toward the long-sought “universal model”
capable of describing the properties of water from the gas to the condensed phases.
66. Development of a “first principles" water potential with flexible monomers. II: Trimer potential energy surface,
third virial coefficient, and small clusters. V. Babin, G.R. Medders, F. Paesani, J. Chem. Theory Comput. 10, 1599
A full-dimensional potential energy function (MB-pol) for simulations of water from the dimer to
bulk phases is developed entirely from “first principles” by building upon the many-body expansion
of the interaction energy. Specifically, the MB-pol potential is constructed by combining a highly
accurate dimer potential energy surface [J. Chem. Theory Comput. 2013, 9, 5395] with explicit
three-body and many-body polarization terms. The three-body contribution, expressed as a
combination of permutationally invariant polynomials and classical polarizability, iimposing the
correct asymptotic behavior as predicted from “first principles”. Here, the accuracy of MB-pol is
demonstrated through comparison of the calculated third virial coefficient with the corresponding
experimental data as well as through analysis of the relative energy differences of small clusters.