Signatures of Nanoconfinement on the Linear and Nonlinear Vibrational Spectroscopy of a Model Hydrogen-Bonded Complex Dissolved in a Polar Solvent

The one-dimensional IR (1D-IR) absorption and IR pump–probe spectra of a hydrogen stretch in a model hydrogen-bonded complex dissolved in a polar solvent confined in spherical hydrophobic cavities of different sizes were simulated using ground-state mixed quantum-classical dynamics. Due to a thorough analysis of key properties of the complex and solvent from equilibrium trajectory data, we were able to gain insight into the microscopic details underlying the spectra. Both the 1D-IR and IR pump–probe spectra manifested the effects of confinement on the relative stabilities of the covalent and ionic forms of the complex through pronounced changes in their peak intensities and numbers. However, in contrast to the 1D-IR spectra, the time-resolved pump–probe spectra were found to be uniquely sensitive to the changes in the molecular dynamics as the cavity size is varied. In particular, it was found that the variations in the time evolutions of the peak intensities in the pump–probe spectra reflect the differences in the solvation dynamics associated with the various forms of the complex in different locations within the cavities. The ability to detect these differences underscores the advantage of using pump–probe spectroscopy for studying nanoconfined systems

Efficient and Deterministic Propagation of Mixed Quantum-Classical Liouville Dynamics

We propose a highly efficient mixed quantum-classical molecular dynamics scheme based on a solution of the quantum-classical Liouville equation (QCLE). By casting the equations of motion for the quantum subsystem and classical bath degrees of freedom onto an approximate set of coupled first-order differential equations for <i>c</i>-numbers, this scheme propagates the composite system in time deterministically in terms of independent classical-like trajectories. To demonstrate its performance, we apply the method to the spin-boson model, a photoinduced electron transfer model, and a Fenna–Matthews–Olsen complex model, and find excellent agreement out to long times with the numerically exact results, using several orders of magnitude fewer trajectories than surface-hopping solutions of the QCLE. Owing to its accuracy and efficiency, this method promises to be very useful for studying the dynamics of mixed quantum-classical systems

Self-Consistent Filtering Scheme for Efficient Calculations of Observables via the Mixed Quantum-Classical Liouville Approach

Over the past decade, several algorithms have been developed for calculating observables using mixed quantum-classical Liouville dynamics, which differ in how accurately they solve the quantum-classical Liouville equation (QCLE). One of these algorithms, known as sequential short-time propagation (SSTP), is a surface-hopping algorithm that solves the QCLE almost exactly, but obtaining converged values of observables requires very large ensembles of trajectories due to the rapidly growing statistical errors inherent to this algorithm. To reduce the ensemble sizes, two filtering schemes (viz., observable cutting and transition filtering) have been previously developed and effectively applied to both simple and complex models. However, these schemes are either ad hoc in nature or require significant trial and error for them to work as intended. In this study, we present a self-consistent scheme, which, in combination with a soundly motivated probability function used for the Monte Carlo sampling of the nonadiabatic transitions, avoids the ad hoc observable cutting and reduces the amount of trial and error required for the transition filtering to work. This scheme is tested on the spin-boson model, in the weak, intermediate, and strong coupling regimes. Our transition filtered results obtained using a newly proposed probability function, which favors the sampling of nonadiabatic transitions with small energy gaps, show a significant improvement in accuracy and efficiency for all coupling regimes over the results obtained using observable cutting and the original implementation of transition filtering and are comparable to those obtained using the combination of these two techniques. It is therefore expected that this novel scheme will substantially reduce ensemble sizes and simplify the computation of observables in more complex systems

The Role of Hydrogen Bonding in the Decomposition of H₂CO₃ in Water: Mechanistic Insights from Ab Initio Metadynamics Studies of Aqueous Clusters

Both concerted and stepwise mechanisms have been proposed for the decomposition of H₂CO₃ in bulk water based on electronic structure and ab initio molecular dynamics calculations. To consistently determine which, if any, mechanism predominates in bulk water, we performed ab initio metadynamics simulations of the decomposition of H₂CO₃ in water clusters of increasing size. We found that, in the small clusters (containing six and nine water molecules), the decomposition occurs according to a concerted proton shuttle mechanism via a cyclic transition state, whereas, in the larger clusters (containing 20 and 45 water molecules), the decomposition occurs according to a two-step mechanism via a solvent-separated HCO₃^–/H₃O⁺ ion pair intermediate. Due to the additional water molecules in the larger clusters, the dissociation of H₂CO₃ into the metastable solvent-separated ion pair was found to be energetically favorable, thereby preventing the formation of the cyclic transition state and committing the decomposition to the sequential route. An analysis of the solvation environment around the H₂CO₃ molecule in the various clusters revealed that the transition from the concerted mechanism to the stepwise mechanism precisely hinges upon the number of water molecules hydrogen bonded to the H₃O⁺ intermediate, which changes as the size of the cluster increases. The larger clusters contain a sufficient number of water molecules to fully solvate the H₃O⁺ intermediate, indicating that they can provide a bulk-like environment for this reaction. Therefore, these results strongly demonstrate that the decomposition of H₂CO₃ in bulk water occurs via the stepwise mechanism