Dynamic Local Structure in Caesium Lead Iodide: Spatial Correlation and Transient Domains.

Metal halide perovskites are multifunctional semiconductors with tunable structures and properties. They are highly dynamic crystals with complex octahedral tilting patterns and strongly anharmonic atomic behavior. In the higher temperature, higher symmetry phases of these materials, several complex structural features are observed. The local structure can differ greatly from the average structure and there is evidence that dynamic 2D structures of correlated octahedral motion form. An understanding of the underlying complex atomistic dynamics is, however, still lacking. In this work, the local structure of the inorganic perovskite CsPbI3 is investigated using a new machine learning force field based on the atomic cluster expansion framework. Through analysis of the temporal and spatial correlation observed during large-scale simulations, it is revealed that the low frequency motion of octahedral tilts implies a double-well effective potential landscape, even well into the cubic phase. Moreover, dynamic local regions of lower symmetry are present within both higher symmetry phases. These regions are planar and the length and timescales of the motion are reported. Finally, the spatial arrangement of these features and their interactions are investigated and visualized, providing a comprehensive picture of local structure in the higher symmetry phases.

Correction: Gauging van der Waals interactions in aqueous solutions of 2D MOFs: when water likes organic linkers more than open-metal sites.

Correction for 'Gauging van der Waals interactions in aqueous solutions of 2D MOFs: when water likes organic linkers more than open-metal sites' by Mohammad R. Momeni et al., Phys. Chem. Chem. Phys., 2021, 23, 3135-3143, https://doi.org/10.1039/D0CP05923D.

Exciton Dissociation in a Model Organic Interface: Excitonic State-Based Surface Hopping versus Multiconfigurational Time-Dependent Hartree.

Quantum dynamical simulations are essential for a molecular-level understanding of light-induced processes in optoelectronic materials, but they tend to be computationally demanding. We introduce an efficient mixed quantum-classical nonadiabatic molecular dynamics method termed eXcitonic state-based Surface Hopping (X-SH), which propagates the electronic Schrödinger equation in the space of local excitonic and charge-transfer electronic states, coupled to the thermal motion of the nuclear degrees of freedom. The method is applied to exciton decay in a 1D model of a fullerene-oligothiophene junction, and the results are compared to the ones from a fully quantum dynamical treatment at the level of the Multilayer Multiconfigurational Time-Dependent Hartree (ML-MCTDH) approach. Both methods predict that charge-separated states are formed on the 10-100 fs time scale via multiple "hot-exciton dissociation" pathways. The results demonstrate that X-SH is a promising tool advancing the simulation of photoexcited processes from the molecular to the true nanomaterials scale.

Modeling energy transfer and absorption spectra in layered metal-organic frameworks based on a Frenkel-Holstein Hamiltonian.

Optimizing energy and charge transfer is key in design and implementation of efficient layered conductive metal-organic frameworks (MOFs) for practical applications. In this work, for the first time, we investigate the role of both long-range excitonic and short-range charge transfer coupling as well as their dependency on reorganization energy on through-space charge transfer in layered MOFs. A π-stacked model system is built based on the archetypal Ni3(HITP)2, HITP = 2,3,6,7,10,11-hexaiminotriphenylene, layered MOF, and a Frenkel/charge transfer Holstein Hamiltonian is developed that takes into account both electronic coupling and intramolecular vibrations. The dependency of the long- and short-range couplings of secondary building units (SBUs) on the stacking geometry is evaluated, which predicts that photophysical properties of layered MOFs critically depend on the degree of ordering between layers. We show that the impact of the two coupling sources in these materials can be discerned or enhanced by the displacement of the SBUs along the long or short molecular axes. The effects of vibronic spectral signatures are examined in both perturbative and resonance regimes. Although, to the best of our knowledge, displacement engineering in layered MOFs currently remains beyond reach, the findings reported here offer new details on the photophysical structure-property relationships in layered MOFs and provide suggestions on how to combine elements of molecular design and engineering to achieve desirable properties and functions for nano- and mesoscale optoelectronic applications.

Metal-to-Semiconductor Transition in Two-Dimensional Metal-Organic Frameworks: An Ab Initio Dynamics Perspective.

Two-dimensional (2D) π-stacked layered metal-organic frameworks (MOFs) are permanently porous and electrically conductive materials with easily tunable crystal structures. Here, we provide an accurate examination of the correlation between structural features and electronic properties of Ni3(HITP)2, HITP = 2,3,6,7,10,11-hexaiminotriphenylene, as an archetypical 2D MOF. The main objective of this work is to unravel the responsive nature of the layered architecture to external stimuli such as temperature and show how the layer flexibility translates to different conductive behaviors. To this end, we employ a combination of quantum mechanical tools, ab initio molecular dynamics (AIMD) simulations, and electronic band structure calculations. We compare the band structure and projected density of states of equilibrated system at 293 K to that of the 0 K optimized structure. Effect of interlayer π-π and intralayer d-π interactions on charge mobility is disentangled and studied by increasing the distance between layers of Ni3(HITP)2 and comparison to an exemplary case of Zn3(HITP)2 2D MOF. Our findings show how a structural change, which can be deformations along the layers, slipping of layers, or change of the interlayer distance, can induce metal-to-semiconductor or indirect-to-direct semiconductor transition, suggesting a way to adjust or even switch between the intralayer vs interlayer conductive anisotropy in Ni3(HITP)2, in particular, and 2D MOFs in general.

Gauging van der Waals interactions in aqueous solutions of 2D MOFs: when water likes organic linkers more than open-metal sites.

Molecular dynamics simulations combined with periodic electronic structure calculations are performed to decipher structural, thermodynamical and dynamical properties of the interfaced vs. confined water adsorbed in hexagonal 1D channels of the 2D layered electrically conductive Cu3(HHTP)2 and Cu3(HTTP)2 metal-organic frameworks (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene and HTTP = 2,3,6,7,10,11-hexathiotriphenylene). Comparing water adsorption in bulk vs. slab models of the studied 2D MOFs shows that water is preferentially adsorbed on the framework walls via forming hydrogen bonds to the organic linkers rather than by coordinating to the coordinatively unsaturated open-Cu2+ sites. Theory predicts that in Cu3(HTTP)2 the van der Waals interactions are stronger which helps the MOF maintain its layered morphology with allowing very little water molecules to diffuse into the interlayer space. Data presented in this work are general and helpful in implementing new strategies for preserving the integrity as well as electrical conductivity of porous materials in aqueous solutions.

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.

Dynamics in the quantum/classical limit based on selective use of the quantum potential.

A classical limit of quantum dynamics can be defined by compensation of the quantum potential in the time-dependent Schrödinger equation. The quantum potential is a non-local quantity, defined in the trajectory-based form of the Schrödinger equation, due to Madelung, de Broglie, and Bohm, which formally generates the quantum-mechanical features in dynamics. Selective inclusion of the quantum potential for the degrees of freedom deemed "quantum," defines a hybrid quantum/classical dynamics, appropriate for molecular systems comprised of light and heavy nuclei. The wavefunction is associated with all of the nuclei, and the Ehrenfest, or mean-field, averaging of the force acting on the classical degrees of freedom, typical of the mixed quantum/classical methods, is avoided. The hybrid approach is used to examine evolution of light/heavy systems in the harmonic and double-well potentials, using conventional grid-based and approximate quantum-trajectory time propagation. The approximate quantum force is defined on spatial domains, which removes unphysical coupling of the wavefunction fragments corresponding to distinct classical channels or configurations. The quantum potential, associated with the quantum particle, generates forces acting on both quantum and classical particles to describe the backreaction.

Excited Li and Na in He(n): influence of the dimer potential energy curves.

The X(2)Σ ground and the A(2)Π and B(2)Σ first two excited states of Li-He and Na-He are determined using high level complete active space self-consistent field-multireference configuration interaction ab initio method. The obtained potentials differ from the ones proposed by Pascale [Phys. Rev. A 28, 632 (1983)], more strongly for the ground than for the excited states. Quantum diffusion Monte Carlo studies of small Li(∗)He(n) and Na(∗)He(n) with n ≤ 5 are performed using a diatomics-in-molecule approach to model the non-pair additive interaction potential. The sensitivity of our results to the A(2)Π and B(2)Σ potentials used is assessed by an analysis of the structure and of the energetics of the clusters. For these small clusters, the physical conclusions are essentially independent of the diatomic curves employed.

Multiple coherent states semiclassical initial value representation spectra calculations of lateral interactions for CO on Cu(100).

Lateral interactions between carbon monoxide molecules adsorbed on a copper Cu(100) surface are investigated via semiclassical initial value representation (SC-IVR) molecular dynamics. A previous analytical potential is extended to include long-range dipole interactions between coadsorbed molecules and preliminary classical simulations were performed to tune the potential parameters. Then, the spectra for several coadsorbed molecules are calculated using the multiple coherent states approximation of the time-averaging representation of the SC-IVR propagator. Results show strong resonances between coadsorbed molecules as observed by past experiments. Resonances turn into dephasing when isotopical substitutions are performed.