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The copper hydroxide ion, CuOH+, serves as the catalytic core in several recently developed water-splitting catalysts, and an understanding of its chemistry is critical to determining viable cataly...

Chemical activation of water by a single chlorine atom was examined computationally for clusters of chlorine radicals and water in a size regime just prior to internal hydration of water/ions, Cl·(H2O)n=4-8,17. This investigation follows a recent analysis of this radical-molecule interaction [Christensen et al. J. Phys. Chem. A 2019, 123, 8657] for n = 1-4, which demonstrated that n = 4 marked a transition in which an oxidized-water structural motif became viable, albeit high in energy. Thousands of unique isomers were computed in the present analysis, which resulted in three structural classes of isomers, including intact hydrated chlorine, hydrogen-transferred (HCl)(OH·)(H2O)n-1, and charge-transferred (Cl-)(H3O+)(OH·)(H2O)n-2 configurations. The electronic structures of these classes were investigated, along with harmonic vibrational signatures that probed the degree of water-network perturbations and generated experimentally verifiable computational predictions. The main outcome of this analysis is that the charge-transferred isomers were stabilized considerably upon increased hydration-leading to an energetic crossover with the hydrogen-transferred forms-but the degree of hydration was surprisingly still not sufficient to achieve crossover between the intact chlorine-water complexes and these charge-separated configurations. Internal hydration of the ions appears to be necessary in order to achieve this separation, which will likely occur at larger cluster sizes.

The gas-phase vibrational spectrum of the isolated iodide–water cluster ion (I–·H2O), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion m...

This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange–correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear–electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an “open teamware” model and an increasingly modular design. This article summarizes technical advances contained in the fifth major release of the Q-Chem quantum chemistry program package, covering developments since 2015. A comprehensive library of exchange-correlation functionals, along with a suite of correlated many-body methods, continues to be a hallmark of the Q-Chem software. The many-body methods include novel variants of both coupled-cluster and configuration-interaction approaches along with methods based on the algebraic diagrammatic construction and variational reduced density-matrix methods. Methods highlighted in Q-Chem 5 include a suite of tools for modeling core-level spectroscopy, methods for describing metastable resonances, methods for computing vibronic spectra, the nuclear-electronic orbital method, and several different energy decomposition analysis techniques. High-performance capabilities including multithreaded parallelism and support for calculations on graphics processing units are described. Q-Chem boasts a community of well over 100 active academic developers, and the continuing evolution of the software is supported by an "open teamware" model and an increasingly modular design.

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl•(H2O)n=1–4. Using an automated isomer-search procedure, dozen...

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.

The isomers of a hydrated Cu(I) ion with n = 1–10 water molecules were investigated by using ab initio quantum chemistry and an automated isomer-search algorithm. The electronic structure and vibrational spectra of the hundreds of resulting isomers were used to analyze the source of the observed bonding patterns. A structural evolution from dominantly two-coordinate structures (n = 1–4) toward a mixture of two- and three-coordinate structures was observed at n = 5–6, where the stability provided by expanded hydrogen-bonding was competitive with the dominantly electrostatic interaction between the water ligand and remaining binding sites of the metal ion. Further hydration (n = 7–10) led to a mixture of three- and four-coordinate structures. The metal ion was found, through spectroscopic signatures, to appreciably perturb the O–H bonds of even third-shell water molecules, which highlighted the ability of this nominally simple ion to partially activate the surrounding water network.

Acceleration of ab initio molecular dynamics (AIMD) simulations can be reliably achieved by extrapolation of electronic data from previous timesteps. Existing techniques utilize polynomial least-squares regression to fit previous steps’ Fock or density matrix elements. In this work, the recursive Burg ‘linear prediction’ technique is shown to be a viable alternative to polynomial regression, and the extrapolation-predicted Fock matrix elements were three orders of magnitude closer to converged elements. Accelerations of 1.8–3.4× were observed in test systems, and in all cases, linear prediction outperformed polynomial extrapolation. Importantly, these accelerations were achieved without reducing the MD integration timestep.

The structures, properties, and spectroscopic signatures of oxidized water clusters,(H2O)+n=6–21, are examined in this work, to provide fundamental insight into renewable energy and radiological processes. Computational quantum chemistry approaches are employed to sample cluster morphologies, yielding hundreds of low-lying isomers with low barriers to interconversion. The ion–radical pair-separation trend, however, which was observed in previous computational studies and in small-cluster spectroscopy experiments, is shown to continue in this larger cluster size regime. The source of this trend is preferential solvation of the hydronium ion by water, including effects beyond the first solvation shell. The fundamental conclusion of this work, therefore, is that the initially formed ion–radical dimer, which has served as a prototypical model of oxidized water, is a nascent species in large, oxidized water clusters and, very likely, bulk water.

The cation dimer of water and hydrogen sulfide, [(H2O)(H2S)]+, serves as a fundamental model for the oxidation chemistry of H2S. The known oxidative metabolism of H2S by biological species in sulfur-rich environments has motivated the study of the inherent properties of this benchmark complex, with possible mechanistic implications for modern water oxidation chemistry. The low-energy isomer of this open-shell ion is a proton-transferred (PT) structure, H3O+⋯SH˙. An alternative PT structure, H3S+⋯OH˙, and a hemibonded (HB) isomer, [H2O·SH2]+, are also stable isomers, placing this complex intermediate to known (H2O)2+ (PT) and (H2S)2+ (HB) limiting regimes. This intermediate character suggested the possibility of unique molecular motion, even in the vibrational ground state. Path integral molecular dynamics and anharmonic vibrational spectroscopy simulations have been performed in this study, in order to understand the inherent quantum molecular motion of this complex. The resulting structural distributions were found to deviate significantly from both classical and harmonic analyses, including the observation of large-amplitude anharmonic motion of the central proton and nearly free rotation of the terminal hydrogens. The predicted vibrational spectra are particularly unique and suggest characteristic signatures of the strong electronic interactions and anharmonic vibrational mode couplings in this radical cation.

A quantitative description of the interactions between ions and water is key to characterizing the role played by ions in mediating fundamental processes that take place in aqueous environments. At the molecular level, vibrational spectroscopy provides a unique means to probe the multidimensional potential energy surface of small ion−water clusters. In this study, we combine the MB-nrg potential energy functions recently developed for ion−water interactions with perturbative corrections to vibrational self-consistent field theory and the local-monomer approximation to disentangle many-body effects on the stability and vibrational structure of the Cs+(H2O)3 cluster. Since several low-energy, thermodynamically accessible isomers exist for Cs+(H2O)3, even small changes in the description of the underlying potential energy surface can result in large differences in the relative stability of the various isomers. Our analysis demonstrates that a quantitative account for three-body energies and explicit treatment of cross-monomer vibrational couplings are required to reproduce the experimental spectrum.

In combined quantum mechanical/molecular mechanical (QM/MM) free energy calculations, it is often advantageous to have a frozen geometry for the quantum mechanical (QM) region. For such multiple-environment single-system (MESS) cases, two schemes are proposed here for estimating the polarization energy: the first scheme, termed MESS-E, involves a Roothaan step extrapolation of the self-consistent field (SCF) energy; whereas the other scheme, termed MESS-H, employs a Newton-Raphson correction using an approximate inverse electronic Hessian of the QM region (which is constructed only once). Both schemes are extremely efficient, because the expensive Fock updates and SCF iterations in standard QM/MM calculations are completely avoided at each configuration. They produce reasonably accurate QM/MM polarization energies: MESS-E can predict the polarization energy within 0.25 kcal/mol in terms of the mean signed error for two of our test cases, solvated methanol and solvated β-alanine, using the M06-2X or ωB97X-D functionals; MESS-H can reproduce the polarization energy within 0.2 kcal/mol for these two cases and for the oxyluciferin-luciferase complex, if the approximate inverse electronic Hessians are constructed with sufficient accuracy.

The water dimer cation, (H2O)2+, has long served as a prototypical reference system for water oxidation chemistry. In spite of this status, a definitive explanation for the anomalous—and dominant—features in the experimental vibrational spectrum [Gardenier, G. H.; Johnson, M. A.; McCoy, A. B. J. Phys. Chem. A, 2009, 113, 4772–4779] has not been determined, and harmonic analyses qualitatively fail to reproduce these features. In this computational study, accurate quantum chemistry methods are combined with a fully coupled, six-dimensional anharmonic model to show that the unassigned bands are the result of resonant mode interactions and strong anharmonic coupling. Such coupling is fundamentally due to the unique electronic structure of this open-shell ion and the manner in which auxiliary modes affect the natural charge-transfer properties of the shared-proton stretch. These unique vibrational signatures provide a key reference point for modern spectroscopic and mechanistic analyses of water-oxidation cata...

Because of both experimental and computational challenges, protonated tryptophan has remained the last aromatic amino acid for which the intrinsic structures of low-energy conformers have not been unambiguously solved. The IR-IR-UV hole-burning spectroscopy technique has been applied to overcome the limitations of the commonly used IR-UV double resonance technique and to measure conformer-specific vibrational spectra of TrpH(+), cooled to T = 10 K. Anharmonic ab initio vibrational spectroscopy simulations unambiguously assign the dominant conformers to the two lowest-energy geometries from benchmark coupled-cluster structure computations. The match between experimental and ab initio spectra provides an unbiased validation of the calculated structures of the two experimentally observed conformers of this benchmark ion. Furthermore, the vibrational spectra provide conformer-specific signatures of the stabilizing interactions, including hydrogen bonding and an intramolecular cation-π interaction.

This work investigates the use of multiple-timestep schemes in imaginary time for computationally efficient ab initio equilibrium path integral simulations of quantum molecular motion. In the simplest formulation, only every n(th) path integral replica is computed at the target level of electronic structure theory, whereas the remaining low-level replicas still account for nuclear motion quantum effects with a more computationally economical theory. Motivated by recent developments for multiple-timestep techniques in real-time classical molecular dynamics, both 1-electron (atomic-orbital basis set) and 2-electron (electron correlation) truncations are shown to be effective. Structural distributions and thermodynamic averages are tested for representative analytic potentials and ab initio molecular examples. Target quantum chemistry methods include density functional theory and second-order Møller-Plesset perturbation theory, although any level of theory is formally amenable to this framework. For a standard two-level splitting, computational speedups of 1.6-4.0x are observed when using a 4-fold reduction in time slices; an 8-fold reduction is feasible in some cases. Multitiered options further reduce computational requirements and suggest that quantum mechanical motion could potentially be obtained at a cost not significantly different from the cost of classical simulations.

Local-mode coordinates have previously been shown to be an effective starting point for anharmonic vibrational spectroscopy calculations. This general approach borrows techniques from localized-orbital machinery in electronic structure theory and generates a new set of spatially localized vibrational modes. These modes exhibit a well-behaved spatial decay of anharmonic mode couplings, which, in turn, allows for a systematic, a priori truncation of couplings and increased computational efficiency. Fully localized modes, however, have been found to lead to unintuitive mixtures of characteristic motions, such as stretches and bends, and accordingly large bilinear couplings. In this work, a very simple, tunable localization frequency window is introduced, in order to realize the transition from normal modes to fully localized modes. Partial localization can be achieved by localizing only pairs of modes within this traveling frequency window, which allows for intuitive interpretation of modes. The optimal window size is suggested to be a few hundreds of wave numbers, based on small- to medium-sized test systems, including water clusters and polypeptides. The new sets of partially localized coordinates retain their spatial coupling decay behavior while providing a reduced number of potential energy evaluations for convergence of anharmonic spectra.