Recent progress in approximate quantum dynamics methods for the study of proton-coupled electron transfer reactions.

Proton-coupled electron transfer (PCET) reactions are ubiquitous natural processes at the heart of energy conversion reactions in photosynthesis and respiration, DNA repair, and diverse enzymatic reactions. Theoretical formulation and computational method developments have eyed modeling of thermal and photoinduced PCET for the last three decades. The accumulation of these studies, collected in dozens of reviews, accounts, and perspectives, has firmly established the influence of quantum effects, including non-adiabatic electronic transitions, vibrational relaxation, zero-point energy, and proton tunneling, on the rate and mechanism of PCET reactions. Here, we focus on some recently-developed methods, spanning the last eight years, that can quantitatively capture these effects in the PCET context and provide efficient means for their qualitative description in complex systems. The theoretical background of each method and their accuracy with respect to exact results are discussed and the results of relevant PCET simulations based on each method are presented.

From ab initio data to high-dimensional potential energy surfaces: A critical overview and assessment of the development of permutationally invariant polynomial potential energy surfaces for single molecules.

The representation of high-dimensional potential energy surfaces by way of the many-body expansion and permutationally invariant polynomials has become a well-established tool for improving the resolution and extending the scope of molecular simulations. The high level of accuracy that can be attained by these potential energy functions (PEFs) is due in large part to their specificity: for each term in the many-body expansion, a species-specific training set must be generated at the desired level of theory and a number of fits attempted in order to obtain a robust and reliable PEF. In this work, we attempt to characterize the numerical aspects of the fitting problem, addressing questions which are of simultaneous practical and fundamental importance. These include concrete illustrations of the nonconvexity of the problem, the ill-conditionedness of the linear system to be solved and possible need for regularization, the sensitivity of the solutions to the characteristics of the training set, and limitations of the approach with respect to accuracy and the types of molecules that can be treated. In addition, we introduce a general approach to the generation of training set configurations based on the familiar harmonic approximation and evaluate the possible benefits to the use of quasirandom sequences for sampling configuration space in this context. Using sulfate as a case study, the findings are largely generalizable and expected to ultimately facilitate the efficient development of PIP-based many-body PEFs for general systems via automation.

Isomeric Equilibria, Nuclear Quantum Effects, and Vibrational Spectra of M+(H2O) n=1-3 Clusters, with M = Li, Na, K, Rb, and Cs, through Many-Body Representations.

A quantitative characterization of the molecular mechanisms that regulate ion solvation is key to the microscopic understanding of fundamental processes taking place in aqueous environments with major implications for different fields, from atmospheric chemistry to materials research and biochemistry. This study presents a systematic analysis of isomeric equilibria for small M+(H2O) n clusters, with M = Li, Na, K, Rb, and Cs, from 0 to 200 K. To determine the relative stability of different isomers of each M+(H2O) n cluster as a function of temperature, replica exchange simulations are carried out at both classical and quantum levels with the recently developed many-body MB-nrg potential energy functions, which have been shown to exhibit chemical accuracy. Anharmonic vibrational spectra are then calculated within the local monomer approximation and found to be in agreement with the available experimental data, providing further support for the accuracy of the MB-nrg potential energy functions. The present analysis indicates that nuclear quantum effects become increasingly important for larger M+(H2O) n clusters containing the heavier alkali metal ions, which is explained in terms of competing ion-water and water-water interactions along with the interplay between energetic and entropic effects. By directly connecting experimental measurements with molecular properties calculated at the quantum mechanical level, this study represents a further step toward the development of a consistent picture of ion hydration from the gas to the condensed phase.

Monitoring Water Clusters "Melt" Through Vibrational Spectroscopy.

Characterizing structural and phase transformations of water at the molecular level is key to understanding a variety of multiphase processes ranging from ice nucleation in the atmosphere to hydration of biomolecules and wetting of solid surfaces. In this study, state-of-the-art quantum simulations with a many-body water potential energy surface, which exhibits chemical and spectroscopic accuracy, are carried out to monitor the microscopic melting of the water hexamer through the analysis of vibrational spectra and appropriate structural order parameters as a function of temperature. The water hexamer is specifically chosen as a case study due to the central role of this cluster in the molecular-level understanding of hydrogen bonding in water. Besides being in agreement with the experimental data available for selected isomers at very low temperature, the present results provide quantitative insights into the interplay between energetic, entropic, and nuclear quantum effects on the evolution of water clusters from "solid-like" to "liquid-like" structures. This study thus demonstrates that computer simulations can now bridge the gap between measurements currently possible for individual isomers at very low temperature and observations of isomer mixtures at ambient conditions.

Assessing the Performance of the Diffusion Monte Carlo Method As Applied to the Water Monomer, Dimer, and Hexamer.

The diffusion Monte Carlo (DMC) method is applied to the water monomer, dimer, and hexamer using q-TIP4P/F, one of the most simple empirical water models with flexible monomers. The bias in the time step (Δτ) and population size (Nw) is investigated. For the binding energies, the bias in Δτ cancels nearly completely, whereas a noticeable bias in Nw remains. However, for the isotope shift (e.g, in the dimer binding energies between (H2O)2 and (D2O)2), the systematic errors in Nw do cancel. Consequently, very accurate results for the latter (within ∼0.01 kcal/mol) are obtained with moderate numerical effort (Nw ∼ 10(3)). For the water hexamer and its (D2O)6 isotopomer, the DMC results as a function of Nw are examined for the cage and prism isomers. For a given isomer, the issue of the walker population leaking out of the corresponding basin of attraction is addressed by using appropriate geometric constraints. The population size bias for the hexamer is more severe, and to maintain accuracy similar to that of the dimer, Nw must be increased by ∼2 orders of magnitude. Fortunately, when the energy difference between the cage and prism is taken, the biases cancel, thereby reducing the systematic errors to within ∼0.01 kcal/mol when using a population of Nw = 4.8 × 10(5) walkers. Consequently, a very accurate result for the isotope shift is also obtained. Notably, both the quantum and isotope effects for the prism-cage energy difference are small.

Self-consistent phonons revisited. II. A general and efficient method for computing free energies and vibrational spectra of molecules and clusters.

The self-consistent phonons (SCP) method provides a consistent way to include anharmonic effects when treating a many-body quantum system at thermal equilibrium. The system is then described by an effective temperature-dependent harmonic Hamiltonian, which can be used to estimate the system's properties, such as its free energy or its vibrational spectrum. The numerical bottleneck of the method is the evaluation of Gaussian averages of the potential energy and its derivatives. Several algorithmic ideas/tricks are introduced to reduce the cost of such integration by orders of magnitude, e.g., relative to that of the previous implementation of the SCP approach by Calvo et al. [J. Chem. Phys. 133, 074303 (2010)]. One such algorithmic improvement is the replacement of standard Monte Carlo integration by quasi-Monte Carlo integration utilizing low-discrepancy sequences. The performance of the method is demonstrated on the calculation of vibrational frequencies of pyrene. It is then applied to compute the free energies of five isomers of water hexamer using the WHBB potential of Bowman and co-workers [J. Chem. Phys. 134, 094509 (2011)]. The present results predict the hexamer prism being thermodynamically most stable, with the free energy of the hexamer cage being about 0.2 kcal mol(-1) higher at all temperatures below T = 200 K.

Mapping the phase diagram for neon to a quantum Lennard-Jones fluid using Gibbs ensemble simulations.

In order to address the issue of whether neon liquid in coexistence with its gas phase can be mapped to a quantum Lennard-Jones (LJ) fluid, we perform a series of simulations using Gibbs ensemble Monte Carlo for a range of de Boer quantum parameters Λ=ℏ/(σ√(mε)). The quantum effects are incorporated by implementing the variational gaussian wavepacket method, which provides an efficient numerical framework for estimating the quantum density at thermal equilibrium. The computed data for the LJ liquid is used to produce its phase diagram as a function of the quantum parameter, 0.065 ≤ Λ ≤ 0.11. These data are then used to fit the experimental phase diagram for neon liquid. The resulting parameters, ε = 35.68 ± 0.03 K and σ = 2.7616 ± 0.0005 Å (Λ = 0.0940), of the LJ pair potential are optimized to best represent liquid neon in coexistence with its gas phase for a range of physically relevant temperatures. This multi-temperature approach towards fitting and assessing a pair-potential is much more consistent than merely fitting a single data point, such as a melting temperature or a second virial coefficient.

A convenient, bio-inspired approach to the synthesis of multi-functional, stable fluorescent silica nanoparticles using poly(ethylene-imine).

Branched poly(ethylene-imine) can be tagged with luminescent dyes (e.g., fluorescein isothiocyanate and tetramethylrhodamine isothiocyanate) and used to precipitate spherical silica particles from 10s-to-100s of nm diameter size under mild conditions. These dye-PEI/SiO2 nanoparticles are highly compatible with polar solvents to give bright fluorescent suspensions, and detailed photophysical characterization reveals well-separated dye moieties with an approximately homogeneous dispersion of dye-PEI conjugate throughout the SiO2 matrix. Reaction of PEI amine groups incorporated at the particle surface affords a simple method for post-synthesis functionalization of these materials, and the formation of FITC/Eosin-Y fluorescence resonance energy transfer pair-tagged particles and SiO2@Au core-shell nanocomposites using this strategy is demonstrated. This bio-inspired approach to multi-functional SiO2 nanoparticles provides a range of potential advantages over traditional "inorganic" syntheses of similar materials, as it proceeds through a scalable, single-step reaction using inexpensive reagents, enables efficient incorporation of luminescent species into the resulting particles with very limited dye aggregation, and provides nanoparticles that do not require post-synthesis modification for further conjugation with species of interest. The method offers a simple means to generate complex nanocomposites, whereby a host of desired species can be incorporated both inside and on the surface of biocompatible SiO2 nanoparticles.

Correction: A convenient, bio-inspired approach to the synthesis of multi-functional, stable fluorescent silica nanoparticles using poly(ethylene-imine).

Correction for 'A convenient, bio-inspired approach to the synthesis of multi-functional, stable fluorescent silica nanoparticles using poly(ethylene-imine)' by Christina A. Bauer, et al., Nanoscale, 2017, 9, 6509-6520.

Anglo-American mothers and the prevention of postpartum depression.

The purpose of this study was to assess for postpartum depression prevention activities of a group of 20 Anglo-American mothers who had given birth within the last year and who denied having postpartum depression. Results of open-ended interviewing and qualitative analysis indicated that in addition to the women being able to articulate a prescribed postpartum period, they also were the beneficiaries of protective measures and rituals, some degree of social seclusion, rest, assistance with tasks, and social recognition of their new social status as a mother. These results underscore the importance of social support for the prevention of postpartum depression and the need to educate mothers, families, and health care workers on their use and value.