91. On the accuracy of the MB-pol many-body potential for water: Interaction energies, vibrational

      frequencies, and classical thermodynamic and dynamical properties from clusters to liquid water and

      ice. S.K. Reddy, S.C. Straight, P. Bajaj, C.H. Pham, M. Riera, D.R. Moberg, M.A. Morales, C. Knight, A. W. Götz,

      F. Paesani, J. Chem. Phys. 145, 194504 (2016). [link]         

The accuracy of MB-pol is systematically assessed across the three phases of water through extensive

comparisons with experimental data and high-level ab initio calculations. Individual many-body contributions

to the interaction energies as well as vibrational spectra of water clusters calculated with MB-pol are in

excellent agreement with reference data obtained at the coupled cluster level. Structural, thermodynamic,

and dynamical properties of the liquid phase at atmospheric pressure calculated as a function of

temperature correctly reproduce the corresponding experimental data. Densities and lattice energies of

several ice phases predicted by MB-pol are also in agreement with experiment. This analysis demonstrates

the high and, in many respects, unprecedented accuracy of MB-pol in representing all phases of water.

90. Spin crossover in the {Fe(pz)[Pt(CN)4]} metal-organic framework upon pyrazine adsorption. C.H. Pham,

      F. Paesani, J. Phys. Chem. Lett. 7, 4022 (2016). [link]        

The spin-crossover behavior of the {Fe(pz)[Pt(CN)4]} metal-organic framework (MOF) upon pyrazine

adsorption is investigated through hybrid Monte Carlo/molecular dynamics (MC/MD) simulations.

In contrast to previous theoretical studies, which re- ported a transition temperature of ~140 K,

the present MC/MD simulations predict that the high-spin state is the most stable state at all

temperatures, in agreement with the experimental observations. The MC/MD simulations also

indicate that the pyrazine molecules adsorbed in the MOF pores lie nearly parallel but staggered

by 60o degree relative to the pyrazine ligands of the framework. The analysis of the magnetization

curve as a function of the temperature demonstrates that the staggered configuration assumed

by the guest pyrazine molecules within the framework is responsible for the stabilization of the

high-spin state. Both the guest pyrazine molecules and the pyrazine ligands of the framework are

effectively locked into the minimum energy configuration and do not display any rotational mobility.

89. Getting the right answers for the right reasons: Toward predictive molecular simulations of water with

      many-body potential energy functions. F. Paesani, Acc. Chem. Res. 49, 1844 (2016). [link]      

This Account provides a critical overview of the performance of the MB-pol many-body potential

energy function through a systematic analysis of energetic, structural, thermodynamic, and dynamical

properties as well as of vibrational spectra of water from the gas to the condensed phase. It is shown

that MB-pol achieves unprecedented accuracy across all phases of water through a quantitative

description of each individual term of the many-body expansion of the interaction energy, with a physically

correct representation of both short- and long-range many-body contributions. Comparisons with experimental

data demonstrate that MB-pol represents a major step toward the long-sought-after “universal model” capable

of accurately describing the molecular properties of water under different conditions and in different environments.

Along this path, future challenges include the extension of the many-body scheme adopted by MB-pol to the

description of generic solutes as well as the integration of MB-pol in an efficient theoretical and computational

framework to model acid-base reactions in solution.

88. i-TTM model for ab initio-based ion-water interaction potentials. II. Alkali metal ion - water potential

     energy functions. M. Riera, A.W. Götz, F. Paesani, Phys. Chem. Chem. Phys. 18, 30334 (2016). [link]    

A new set of i-TTM potential energy functions describing the interactions between alkali metal ions

and water molecules is reported. Following our previous study of halide ion-water interactions

[J. Phys. Chem. B 120, 1822 (2016)], the new i-TTM potentials are derived from fits to CCSD(T)

reference energies and, by construction, are compatible with the MB-pol many-body potential,

which has been shown to accurately predict the properties of water from the gas to the condensed

phase. Within the i-TTM formalism, two-body repulsion, electrostatic, and dispersion energies are

treated explicitly, while many-body effects are represented by classical induction. The accuracy of

the new i-TTM potentials is assessed through extensive comparisons with results obtained from

different ab initio methods, including CCSD(T), CCSD(T)-F12b, DF-MP2, and several DFT models,

as well as from polarizable force fields for M+(H2O)n clusters with M+ = Li+, Na+, K+, Rb+, and Cs+,

and n = 1 − 4.

87. Modeling molecular interactions in water: From pairwise to many-body potential energy functions.

      G.A. Cisneros, K.T. Wikfeldt, L. Ojamäe, J. Lu, Y. Xu, H. Torabifard, A. Bartók, C. Csányi, V. Molinero, F. Paesani,

      Chem. Rev. 116, 7501 (2016). [link]  

In this article, we review the recent progress in the development of analytical potential energy

functions that aim at correctly representing many-body effects. Starting from the many-body expansion

of the interaction energy, specific focus is on different classes of potential energy functions built upon

a hierarchy of approximations and on their ability to accurately reproduce reference data obtained from

state-of-the-art electronic structure calculations and experimental measurements. We show that most

recent potential energy functions, which include explicit short-range representations of two-body and

three-body effects along with a physically correct description of many-body effects at all distances,

predict the properties of water from the gas to the condensed phase with unprecedented accuracy, thus

opening the door to the long-sought “universal model” capable of describing the behavior of water under

different conditions and in different environments.

86. Towards chemical accuracy in the description of ion-water interactions through many-body representations.     

      I. Halide-water dimer potential energy surfaces. P. Bajaj, A.W. Götz, F. Paesani, J. Chem. Theory Comput. 12, 2698

     (2016). [link]  

We report the development of full-dimensional many-body potential energy functions, called MB-nrg

(Many-Body-energy), for molecular simulations of halide ion-water systems from the gas to the condensed

phase. The MB-nrg potentials are derived entirely from "first principles" calculations carried out at the

F12 explicitly correlated coupled-cluster level including single, double and perturbative triple excitations,

CCSD(T)-F12, in the complete basis set limit. Building upon the functional form of the MB-pol water

potential, the MB-nrg potentials are expressed through the many-body expansion of the total energy in

terms of explicit contributions representing one-body, two-body, and three-body interactions, with all

higher-order contributions being described by classical induction. The accuracy of the MB-nrg potentials

is systematically assessed through extensive comparisons with results obtained using both ab initio

models and polarizable force fields for energies, structures, and harmonic frequencies of the H2O-X- dimers.

85. Molecular mechanisms of spin crossover in the {Fe(pz)[Pt(CN)4]} metal-organic framework upon water

      adsorption. C.H. Pham, J. Cirera, F. Paesani, J. Am. Chem. Soc. 138, 6123 (2016). [link]  

The rational design of multifunctional materials with properties that can be selectively controlled at the

molecular level is key to the development and application of nanoscale devices. In this study, molecular

dynamics simulations using ligand-field molecular mechanics are performed to elucidate, for the first

time, the molecular mechanisms responsible for the variation of the spin-crossover properties of the

{Fe(pz)[Pt(CN)4]} metal-organic framework upon water adsorption. The simulations demonstrate a

direct relationship between the water loading adsorbed in the pores and the variation of the

spin-crossover transition temperature, with the high-spin state of the material becoming gradually more

stabilized as the number of adsorbed water molecules increases. The decrease of the spin-crossover

temperature of {Fe(pz)[Pt(CN)4]} upon water adsorption predicted by the simulations is in agreement with

the available experimental data and is traced back to the elongation of the bonds between the Fe(II)

atoms and the organic linkers induced by interactions of the adsorbed water molecules with the framework.

84. Exploring electrostatic effects on the hydrogen bond network of liquid water through many-body molecular  

      dynamics. S.C. Straight, F. Paesani, J. Phys. Chem. B 120, 8539 (2016). [link]  

Linear and nonlinear infrared spectra of dilute HOD in H2O are computed from many-body molecular dynamics

simulations with the MB-pol potential, which have been shown to accurately predict the properties of water

from the gas to the condensed phase. The effects of various approximations to the many-body expansion

of the dipole moment surface on the OD-stretch absorption line shapes are analyzed at different levels of

theory. The interplay between effects associated with the variation of the HOD dipole moment and

instantaneous nuclear configurations causes qualitative differences in the absorption profiles, which are

traced back to how induction contributions are treated within the many-body formalism. Further analysis of

the multidimensional infrared spectra demonstrates that the spectral diffusion of the OD stretching frequencies

depends explicitly on the level of truncation in the many-body expansion of the dipole moment in the

short-time regime that is associated with intact hydrogen-bond dynamics.

83. Dissecting the molecular structure of the air/water interface from quantum simulations of the sum-frequency

      generation spectrum. G.R. Medders, F. Paesani, J. Am. Chem. Soc. 138, 3912 (2016). [link]  

In this study, we demonstrate that the application of quantum many-body molecular dynamics (MB-MD),

which enables spectroscopically accurate simulations of water from the gas to the condensed phase,

leads to a definitive molecular-level picture of the interface region. For the first time, excellent

agreement is obtained between the simulated vibrational sum-frequency generation spectrum and the

most recent state-of-the-art measurements, without requiring any empirical frequency shift or ad hoc

scaling of the spectral intensity. A systematic dissection of the spectral features demonstrates that a

rigorous representation of nuclear quantum effects as well as of many-body contributions is necessary

for a quantitative reproduction of the experimental data. The unprecedented accuracy of the theoretical

spectra indicates that MB-MD can enable predictive studies of aqueous interfaces,  providing unique

insights into fundamental processes of relevance in chemistry, biology, materials science, and

environmental research.

82. The effects of framework dynamics on the behavior of water adsorbed in the [Zn(l-L)(Cl)] and Co-MOF-74

      metal-organic frameworks. . Z.L. Terranova, F. Paesani, Phys. Chem. Chem. Phys. 18, 8196 (2016). [link]  

The effects of framework flexibility on the properties of water adsorbed in two prototypical

metal-organic frameworks are investigated through molecular dynamics simulations. It is found that

water in a flexible model of [Zn(l-L)(Cl)] exhibit slower dynamics than when the framework is artificially

held rigid in the simulations. In contrast, the water dynamics in Co-MOF-74 is predicted to be

accelerated by the framework vibrations. The origin of this different behavior directly relates to how

water interacts with the two frameworks. While water molecules in [Zn(l-L)(Cl)] donate a single

hydrogen bond to the Zn-Cl groups, water molecules in Co-MOF-74 initially bind to the Co atoms of

the framework via their oxygen atoms, thus leaving each molecule free to form two hydrogen bonds

with additional molecules adsorbed at higher loading. The simulation results indicate that taking into

account the framework flexibility in computer simulations is necessary for a quantitative modeling of

adsorption and transport processes in metal–organic frameworks.

81. Proton transport in a highly conductive porous zirconium-based metal-organic framework: Molecular insight.

      D. Damasceno Borges, S. Devautour-Vinot, H. Jobic, J. Olivier, F. Nouar, R. Semino, T. Devic, C. Serre, F. Paesani,

      G. Maurin, Angew. Chem. Int. Ed. 55, 3919 (2016). [link]  

The water stable UiO-66(Zr)-(CO2H)2 MOF exhibits a superprotonic conductivity of 2.3x10-3 S cm-1 at 90 °C

and 95 % relative humidity. Quasi-elastic neutron scattering measurements combined with aMS-EVB3

molecular dynamics simulations were able to probe individually the dynamics of both confined protons

and water molecules and to further reveal that the proton transport is assisted by the formation of a

hydrogen-bonded water network that spans from the tetrahedral to the octahedral cages of this MOF.

This is the first joint experimental/modeling study that unambiguously elucidates the proton-conduction

mechanism at the molecular level in a highly conductive MOF.

80. i-TTM model for ab initio-based ion-water interaction potentials. 1. Halide-water potential energy functions.

      D.J. Arismendi-Arrieta, M. Riera, P. Bajaj, R. Prosmiti, F. Paesani, J. Phys. Chem. B 120, 1822 (2016). [link]  

New potential energy functions (i-TTM) describing the interactions between halide ions and

water molecules are reported. The i-TTM potentials are derived from fits to electronic structure data

and include an explicit treatment of two-body repulsion, electrostatics, and dispersion energy.

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, which has recently been shown to accurately predict the properties of water

from the gas to the condensed phase. The accuracy of the i-TTM potentials is assessed through

extensive comparisons with CCSD(T)-F12, DF-MP2, and DFT data as well as with results obtained

with common polarizable force fields for X−(H2O)n clusters with X− = F−, Cl−, Br−, and I−, and

n = 1 − 8. By construction, the new i-TTM potentials will enable direct simulations of vibrational

spectra of halide-water systems from clusters to bulk and interfaces.

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