Anomalies and Local Structure of Liquid Water from Boiling to the Supercooled Regime as Predicted by the Many-Body MB-pol Model.

For the past 50 years, researchers have sought molecular models that can accurately reproduce water's microscopic structure and thermophysical properties across broad ranges of its complex phase diagram. Herein, molecular dynamics simulations with the many-body MB-pol model are performed to monitor the thermodynamic response functions and local structure of liquid water from the boiling point down to deeply supercooled temperatures at ambient pressure. The isothermal compressibility and isobaric heat capacity show maxima near 223 K, in excellent agreement with recent experiments, and the liquid density exhibits a minimum at ∼208 K. A local tetrahedral arrangement, where each water molecule accepts and donates two hydrogen bonds, is found to be the most probable hydrogen-bonding topology at all temperatures. This work suggests that MB-pol may provide predictive capability for studies of liquid water's physical properties across broad ranges of thermodynamic states, including the so-called water's "no man's land" which is difficult to probe experimentally.

Configuration interaction singles with spin-orbit coupling: Constructing spin-adiabatic states and their analytical nuclear gradients.

For future use in modeling photoexcited dynamics and intersystem crossing, we calculate spin-adiabatic states and their analytical nuclear gradients within configuration interaction singles theory. These energies and forces should be immediately useful for surface hopping dynamics, which are natural within an adiabatic framework. The resulting code has been implemented within the Q-Chem software and preliminary results suggest that the additional cost of including spin-orbit coupling within the singles-singles block is not large.

Ultrafast Electronic Relaxation through a Conical Intersection: Nonadiabatic Dynamics Disentangled through an Oscillator Strength-Based Diabatization Framework.

We employ surface hopping trajectories to model the short-time dynamics of gas-phase and partially solvated 4-(N,N-dimethylamino)benzonitrile (DMABN), a dual fluorescent molecule that is known to undergo a nonadiabatic transition through a conical intersection. To compare theory vs time-resolved fluorescence measurements, we calculate the mixed quantum-classical density matrix and the ensemble averaged transition dipole moment. We introduce a diabatization scheme based on the oscillator strength to convert the TDDFT adiabatic states into diabatic states of La and Lb character. Somewhat surprisingly, we find that the rate of relaxation reported by emission to the ground state is almost 50% slower than the adiabatic population relaxation. Although our calculated adiabatic rates are largely consistent with previous theoretical calculations and no obvious effects of decoherence are seen, the diabatization procedure introduced here enables an explicit picture of dynamics in the branching plane, raising tantalizing questions about geometric phase effects in systems with dozens of atoms.

Dissecting the Molecular Structure of the Air/Water Interface from Quantum Simulations of the Sum-Frequency Generation Spectrum.

The molecular characterization of the air/water interface is a key step in understanding fundamental multiphase phenomena ranging from heterogeneous chemical processes in the atmosphere to the hydration of biomolecules. The apparent simplicity of the air/water interface, however, masks an underlying complexity associated with the dynamic nature of the water hydrogen-bond network that has so far hindered an unambiguous characterization of its microscopic properties. Here, we demonstrate that the application of quantum many-body molecular dynamics, 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 energy and electrostatic contributions is necessary for a quantitative reproduction of the experimental data. The unprecedented accuracy of the simulations presented here indicates that quantum many-body molecular dynamics can enable predictive studies of aqueous interfaces, which by complementing analogous experimental measurements will provide unique molecular insights into multiphase and heterogeneous processes of relevance in chemistry, biology, materials science, and environmental research.

Infrared and Raman Spectroscopy of Liquid Water through "First-Principles" Many-Body Molecular Dynamics.

Vibrational spectroscopy is a powerful technique to probe the structure and dynamics of water. However, deriving an unambiguous molecular-level interpretation of the experimental spectral features remains a challenge due to the complexity of the underlying hydrogen-bonding network. In this contribution, we present an integrated theoretical and computational framework (named many-body molecular dynamics or MB-MD) that, by systematically removing uncertainties associated with existing approaches, enables a rigorous modeling of vibrational spectra of water from quantum dynamical simulations. Specifically, we extend approaches used to model the many-body expansion of interaction energies to develop many-body representations of the dipole moment and polarizability of water. The combination of these "first-principles" representations with centroid molecular dynamics simulations enables the simulation of infrared and Raman spectra of liquid water under ambient conditions that, without relying on any ad hoc parameters, are in good agreement with the corresponding experimental results. Importantly, since the many-body energy, dipole, and polarizability surfaces employed in the simulations are derived independently from accurate fits to correlated electronic structure data, MB-MD allows for a systematic analysis of the calculated spectra in terms of both electronic and dynamical contributions. The present analysis suggests that, while MB-MD correctly reproduces both the shifts and the shapes of the main spectroscopic features, an improved description of quantum dynamical effects possibly combined with a dissociable water potential may be necessary for a quantitative representation of the OH stretch band.

On the representation of many-body interactions in water.

Recent work has shown that the many-body expansion of the interaction energy can be used to develop analytical representations of global potential energy surfaces (PESs) for water. In this study, the role of short- and long-range interactions at different orders is investigated by analyzing water potentials that treat the leading terms of the many-body expansion through implicit (i.e., TTM3-F and TTM4-F PESs) and explicit (i.e., WHBB and MB-pol PESs) representations. It is found that explicit short-range representations of 2-body and 3-body interactions along with a physically correct incorporation of short- and long-range contributions are necessary for an accurate representation of the water interactions from the gas to the condensed phase. Similarly, a complete many-body representation of the dipole moment surface is found to be crucial to reproducing the correct intensities of the infrared spectrum of liquid water.

On the interplay of the potential energy and dipole moment surfaces in controlling the infrared activity of liquid water.

Infrared vibrational spectroscopy is a valuable tool for probing molecular structure and dynamics. However, obtaining an unambiguous molecular-level interpretation of the spectral features is made difficult, in part, due to the complex interplay of the dipole moment with the underlying vibrational structure. Here, we disentangle the contributions of the potential energy surface (PES) and dipole moment surface (DMS) to the infrared spectrum of liquid water by examining three classes of models, ranging in complexity from simple point charge models to accurate representations of the many-body interactions. By decoupling the PES from the DMS in the calculation of the infrared spectra, we demonstrate that the PES, by directly modulating the vibrational structure, primarily controls the width and position of the spectroscopic features. Due to the dependence of the molecular dipole moment on the hydration environment, many-body electrostatic effects result in a ∼100 cm(-1) redshift in the peak of the OH stretch band. Interestingly, while an accurate description of many-body collective motion is required to generate the correct (vibrational) structure of the liquid, the infrared intensity in the OH stretching region appears to be a measure of the local structure due to the dominance of the one-body and short-ranged two-body contributions to the total dipole moment.

Water Dynamics in Metal-Organic Frameworks: Effects of Heterogeneous Confinement Predicted by Computational Spectroscopy.

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.

Development of a "First Principles" Water Potential with Flexible Monomers. II: Trimer Potential Energy Surface, Third Virial Coefficient, and Small Clusters.

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, is derived from a fit to more than 12000 three-body energies calculated at the CCSD(T)/aug-cc-pVTZ level of theory, imposing 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.

Development of a "First-Principles" Water Potential with Flexible Monomers. III. Liquid Phase Properties.

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.

Many-Body Convergence of the Electrostatic Properties of Water.

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 essentially negligible.

A Critical Assessment of Two-Body and Three-Body Interactions in Water.

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. Unfortunately, such ab initio calculations are severely limited by system size. 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.

Cancer chemotherapeutic agents as human teratogens.

BACKGROUND: Cancer is the second leading cause of death among women of reproductive age. Although the coincidence of pregnancy and cancer is rare and treatment may sometimes be safely delayed, the use of chemotherapeutic agents in pregnancy is sometimes unavoidable or inadvertent. METHODS: We review the literature for the use of antineoplastic agents in single-agent and combination therapy from 1951 through June 2012. We also summarize the evidence relating to teratogenicity of disorder-specific combination chemotherapy treatments for those malignancies frequently encountered in women of childbearing age. Major endpoints were called "adverse pregnancy outcomes" (APOs), to include structural anomalies (congenital malformations), functional defects, blood or electrolyte abnormalities, stillbirths, spontaneous abortions (miscarriages), and fetal, neonatal, or maternal deaths. RESULTS: The registry totals 863 cases. Rates of APOs (and congenital malformations) after any exposure were 33% (16%), 27% (8%), and 25% (6%), for first, second, and third trimesters. Among the groups of cancer drugs, antimetabolites and alkylating agents have the highest rates of APOs. Mitotic inhibitors and antibiotics seem more benign. Mixed results were observed from single-agent exposure, often because of small numbers of exposures. As a whole, the alkylating agents and antimetabolites are more harmful when given as a single agent rather than as part of a regimen. First-trimester exposure poses a more permanent risk to the fetus. CONCLUSIONS: Systematic ascertainment of women early in pregnancy, preferably in a population base, is needed for assessment of true risks. Long-term follow-up is needed to rule out neurobehavioral effects.

Toward a Universal Water Model: First Principles Simulations from the Dimer to the Liquid Phase.

A full-dimensional model of water, HBB2-pol, derived entirely from "first-principles", is introduced and employed in computer simulations ranging from the dimer to the liquid. HBB2-pol provides excellent agreement with the measured second and third virial coefficients and, by construction, reproduces the dimer vibration-rotation-tunneling spectrum. The model also predicts the relative energy differences between isomers of small water clusters within the accuracy of highly correlated electronic structure methods. Importantly, when combined with simulation methods that explicitly include zero-point energy and quantum thermal motion, HBB2-pol accurately describes both structural and dynamical properties of the liquid phase.