The Near-Sightedness of Many-Body Interactions in Anharmonic Vibrational Couplings.

Couplings between vibrational motions are driven by electronic interactions, and these couplings carry special significance in vibrational energy transfer, multidimensional spectroscopy experiments, and simulations of vibrational spectra. In this investigation, the many-body contributions to these couplings are analyzed computationally in the context of clathrate-like alkali metal cation hydrates, including Cs(HO), Rb(HO), and K(HO), using both analytic and quantum-chemistry potential energy surfaces. Although the harmonic spectra and one-dimensional anharmonic spectra depend strongly on these many-body interactions, the mode-pair couplings were, perhaps surprisingly, found to be dominated by one-body effects, even in cases of couplings to low-frequency modes that involved the motion of multiple water molecules.

IR spectroscopic characterization of [M,C,2H] (M = Ru and Rh) products formed by reacting 4d transition metal cations with oxirane: Spectroscopic evidence for multireference character in RhCH.

A combination of infrared multiple-photon dissociation (IRMPD) action spectroscopy and quantum chemical calculations was employed to investigate the [M,C,2H] (M = Ru and Rh) species. These ions were formed by reacting laser ablated M ions with oxirane (ethylene oxide, c-CHO) in a room-temperature ion trap. IRMPD spectra for the Ru species exhibit one major band and two side bands, whereas spectra for the Rh species contain more distinct bands.

Pitfalls in the n-mode representation of vibrational potentials.

Simulations of anharmonic vibrational motion rely on computationally expedient representations of the governing potential energy surface. The n-mode representation (n-MR)-effectively a many-body expansion in the space of molecular vibrations-is a general and efficient approach that is often used for this purpose in vibrational self-consistent field (VSCF) calculations and correlated analogues thereof. In the present analysis, a lack of convergence in many VSCF calculations is shown to originate from negative and unbound potentials at truncated orders of the n-MR expansion.

Accelerating and stabilizing the convergence of vibrational self-consistent field calculations via the direct inversion of the iterative subspace (vDIIS) algorithm.

The vibrational self-consistent field (VSCF) method yields anharmonic states and spectra for molecular vibrations, and it serves as the starting point for more sophisticated correlated-vibration methods. Convergence of the iterative, non-linear optimization in VSCF calculations can be erratic or altogether unsuccessful, particularly for chemical systems involving low-frequency motions. In this work, a vibrational formulation of the Direct Inversion of the Iterative Subspace method of Pulay is presented and investigated.

Persistence of a Delocalized Radical in Larger Clusters of Hydrated Copper(II) Hydroxide, CuOH(HO).

The structures, vibrational spectra, and electronic properties of copper hydroxide hydrates CuOH(HO) were investigated with quantum chemistry computations. As a follow-up to a previous analysis of CuOH(HO), this investigation examined the progression as the square-planar metal coordination environment was filled and as solvation shells expanded. Four-, five-, and six-coordinate structures were found to be low-energy isomers.

Accelerating Anharmonic Spectroscopy Simulations via Local-Mode, Multilevel Methods.

computer simulations of anharmonic vibrational spectra provide nuanced insight into the vibrational behavior of molecules and complexes. The computational bottleneck in such simulations, particularly for potentials, is often the generation of mode-coupling potentials. Focusing specifically on two-mode couplings in this analysis, the combination of a local-mode representation and multilevel methods is demonstrated to be particularly symbiotic.

Structural, Thermodynamic, and Spectroscopic Evolution in the Hydration of Copper(II) Ions, Cu(HO).

Gas-phase clusters of the hydrated Cu(II) cation with 2-8 water molecules were investigated using ab initio quantum chemistry. Isomer structures, energies, and vibrational spectra were computed across this size range, yielding a qualitative picture of this ion as an intact Cu hydrate that also partially oxidizes the surrounding water network at equilibrium. At sufficient cluster sizes, these ion hydrates also become thermodynamically preferred over competitive Cu(II) hydroxide hydrates.

Molecular Motion in the Interconverting σ-H, Di-, and Tri-hydride Regimes: .

The transition-metal complex is known to exist in three possible isomeric forms, including a nonclassical, σ-bound dihydrogen complex and two classical dihydride isomers. As such, it has served as a model complex for the energies of conversion between these limiting structural regimes. In the present study, ab initio molecular dynamics computer simulations, combined with enhanced sampling techniques, were utilized to directly assess the degree of motion and isomerization of the dihydrogen/dihydride moieties in this complex.

Software for the frontiers of quantum chemistry: An overview of developments in the Q-Chem 5 package.

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.

Adiabatic Molecular Orbital Tracking in Molecular Dynamics.

The ab initio molecular dynamics (AIMD) method provides a computational route for the real-time simulation of reactive chemistry. An often-overlooked capability of this approach is the opportunity to examine the electronic evolution of a chemical system. For AIMD trajectories based on Hartree-Fock or density functional theory methods, the real-time evolution of single-particle molecular orbitals (MOs) can provide detailed insights into the time-dependent electronic structure of molecules.

Electronic Structure and Vibrational Signatures of the Delocalized Radical in Hydrated Clusters of Copper("II") Hydroxide CuOH(HO).

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 catalytic mechanisms. In spite of its importance, the electronic structure of this open-shell ion has remained ambiguous in the literature. In particular, computed values for both the thermodynamics of hydration and the vibrational signatures of the mono- and dihydrates have shown prohibitively large errors compared to values from recent experimental measurements.

Infrared signatures of isomer selectivity and symmetry breaking in the Cs(HO) complex using many-body potential energy functions.

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(HO) cluster.

Stepwise Activation of Water by Open-Shell Interactions, Cl(HO).

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(HO). This investigation follows a recent analysis of this radical-molecule interaction [Christensen et al. 2019, 123, 8657] for = 1-4, which demonstrated that = 4 marked a transition in which an oxidized-water structural motif became viable, albeit high in energy.

Spectroscopic Signatures of Mode-Dependent Tunnel Splitting in the Iodide-Water Binary Complex.

The gas-phase vibrational spectrum of the isolated iodide-water cluster ion (I·HO), first reported in 1996, presents one of the most difficult, long-standing spectroscopic puzzles involving ion microhydration. Although the spectra of the smaller halides are well described in the context of an asymmetrical ground-state structure in which only one OH group is hydrogen-bonded to the ion, the I·HO spectrum displays multiplet structures with partially resolved rotational patterns that are additionally influenced by quantum nuclear spin statistics. In this study, this complex behavior is unraveled with a combination of experimental methods, including ion preparation in a temperature-controlled ion trap and spectral simplification through applications of tag-free, two-color IR-IR double-resonance spectroscopy.

Probing the Partial Activation of Water by Open-Shell Interactions, Cl(HO).

The partial chemical activation of water by reactive radicals was examined computationally for small clusters of chlorine and water, Cl(HO). Using an automated isomer-search procedure, dozens of unique, stable structures were computed. Among the resulting structural classes were intact, hydrated-chlorine isomers, as well as hydrogen-abstracted (HCl)(OH)(HO) configurations.

Nuclear Motion in the Intramolecular Dihydrogen-Bound Regime of an Aminoborane Complex.

The 1,3-diaza-2,4-diborobutane (NBNB) molecule serves as the smallest model complex of an intramolecular "dihydrogen bond," which involves a nominally hydrogen-bonding interaction between amine and borane hydrogen atoms. In the present study, the role of this dihydrogen bond in influencing the inherent molecular dynamics of NBNB is investigated computationally with ab initio molecular dynamics and path integral molecular dynamics techniques, as well as vibrational spectra analysis and static quantum chemistry computations. These simulations indicate that the dihydrogen-bonding interaction impacts both the high- and low-frequency motions of the molecule, with the dominant motions involving low-frequency backbone isomerization and terminal amine rotation.

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.

Signatures of Size-Dependent Structural Patterns in Hydrated Copper(I) Clusters, Cu(HO).

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.

Tuning vibrational mode localization with frequency windowing.

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.

Vibrational Signatures of Conformer-Specific Intramolecular Interactions in Protonated Tryptophan.

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.

Accelerating Ab Initio Path Integral Simulations via Imaginary Multiple-Timestepping.

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.

Consecutive Charging of a Molecule-on-Insulator Ensemble Using Single Electron Tunnelling Methods.

We present the local charge state modification at room temperature of small insulator-supported molecular ensembles formed by 1,1'-ferrocenedicarboxylic acid on calcite. Single electron tunnelling between the conducting tip of a noncontact atomic force microscope (NC-AFM) and the molecular islands is observed. By joining NC-AFM with Kelvin probe force microscopy, successive charge build-up in the sample is observed from consecutive experiments.

Multiple-Time Step Ab Initio Molecular Dynamics Based on Two-Electron Integral Screening.

A multiple-timestep ab initio molecular dynamics scheme based on varying the two-electron integral screening method used in Hartree-Fock or density functional theory calculations is presented. Although screening is motivated by numerical considerations, it is also related to separations in the length- and timescales characterizing forces in a molecular system: Loose thresholds are sufficient to describe fast motions over short distances, while tight thresholds may be employed for larger length scales and longer times, leading to a practical acceleration of ab initio molecular dynamics simulations. Standard screening approaches can lead, however, to significant discontinuities in (and inconsistencies between) the energy and gradient when the screening threshold is loose, making them inappropriate for use in dynamics.