Journal of Chemical Physics最新文献

Reduced density matrix functional theory (RDMFT) and coupled cluster theory restricted to paired double excitations (pCCD) are emerging as efficient methodologies for accounting for the so-called non-dynamic electronic correlation effects. Up to now, molecular calculations have been performed with real-valued orbitals. However, before extending the applicability of these methodologies to extended systems, where Bloch states are employed, the subtleties of working with complex-valued orbitals and the consequences of imposing time-reversal symmetry must be carefully addressed. In this work, we describe the theoretical and practical implications of adopting time-reversal symmetry in RDMFT and pCCD when allowing for complex-valued orbital coefficients. The theoretical considerations primarily affect the optimization algorithms, while the practical implications raise fundamental questions about the stability of solutions. In particular, we find that complex solutions lower the energy when non-dynamic electronic correlation effects are pronounced. We present numerical examples to illustrate and discuss these instabilities and possible problems introduced by N-representability violations.

Molecular dynamics simulations were conducted using the generalized replica exchange method (gREM) on the 4-cyano-4'-n-alkyl biphenyl (nCB) system with n = 5, 6, 7, and 8, which exhibits a nematic-isotropic (NI) phase transition. Sampling near the phase transition temperature in systems undergoing first-order phase transitions, such as the NI phase transition, is demanding due to the substantial energy gap between the two phases. To address this, gREM, specifically designed for first-order phase transitions, was utilized to enhance sampling near the NI phase transition temperature. Free-energy calculations based on the energy representation (ER) theory were employed to characterize the NI phase transition. ER evaluates the insertion free energy of the nCB molecule for both nematic and isotropic phases, revealing a change in the temperature dependence across the NI phase transition. Further decomposition into energetic and entropic terms quantitatively shows the balance between these contributions at the NI phase transition temperature.

Due to their unique physiochemical properties that may be tailored for specific purposes, ionic liquids (ILs) have been investigated for various applications, including chemical separations, catalysis, energy storage, and space propulsion. The different cations and anions comprising ILs may be selected to optimize a range of desired properties, such as thermal stability, ionic conductivity, and volatility, leading to the designation of certain ILs as designer "green" solvents. The effect of counterions on the properties of ILs is of both fundamental scientific interest and technological importance. Herein, we report a systematic experimental and theoretical investigation of the size, charge, stability toward dissociation, and geometric/electronic structure of 1-ethyl-3-methyl imidazolium (EMIM)-based IL clusters containing two different atomic counterions (i.e., bromide [Br-] and iodide [I-]). This work extends our studies of EMIM+ cations with atomic chloride (Cl-) and molecular tetrafluoroborate (BF4-) anions reported previously by Baxter et al. [Chem. Mater. 34, 2612 (2022)] and Zhang et al. [J. Phys. Chem. Lett. 11, 6844 (2020)], respectively. Distributions of anionic IL clusters were generated in the gas phase using electrospray ionization and characterized by high mass resolution mass spectrometry, energy-resolved collision-induced dissociation, and negative ion photoelectron spectroscopy experiments. The experimental results reveal anion-dependent trends in the size distribution, relative abundance, ionic charge state, stability toward dissociation, and electron binding energies of the IL clusters. Complementary global optimization theory provides molecular-level insights into the bonding and electronic structure of a selected subset of clusters, including their low energy structures and electrostatic potential maps, and how these fundamental characteristics are influenced by anion substitution. Collectively, our findings demonstrate how the fundamental properties of ILs, which determine their suitability for many applications, may be tuned by substituting counterions. These observations are critical in the sub-nanometer cluster size regime where phenomena do not scale predictably to the bulk phase, and each atom counts toward determining behavior.

Ice nucleation and growth are critical in many fields, including atmospheric science, cryobiology, and aviation. However, understanding the detailed mechanisms of ice crystal growth remains challenging. In this work, crystallization at the ice/quasi-liquid layer (QLL) interface of the basal and primary prism (prism1) surfaces of hexagonal ice (Ih) was investigated using molecular dynamics simulations across a wide range of temperatures for the TIP4P/Ice model, with comparisons to the mW coarse-grained model. Together with elucidating the temperature-dependent mechanisms of crystallization, face-specific growth rates were systematically estimated. While the prism surface generally exhibits faster growth rates than the basal surface, a temperature-dependent crossover in growth rates between the basal and prism surfaces is observed in TIP4P/Ice simulations, which correlates with crossovers in QLL thickness and properties and with the well-known column to platelets transition in ice-crystal habits at low vapor pressure. This observation helps decode the complex dependence between crystal morphology and temperature in ice crystals.

Self-assembling amphiphilic cyanine dyes, such as C8S3, are promising candidates for energy storage and optoelectronic applications due to their efficient energy transport properties. C8S3 is known to self-assemble in water into double-walled J-aggregates. Thus far, the molecular self-assembly steps remain shrouded in mystery. Here, we employ a multiscale approach to unravel the first self-assembly step: dimerization. Our multiscale approach combines molecular dynamics simulations with quantum chemistry calculations to obtain a Frenkel exciton Hamiltonian, which we then use in spectral calculations to determine the absorption and two-dimensional electronic spectra of C8S3 monomer and dimer systems. We model these systems solvated in both water and methanol, validating our model with experiments in methanol solution. Our theoretical results predict a measurable anisotropy decay upon dimerization, which is experimentally confirmed. Our approach provides a tool for the experimental probing of dimerization. Moreover, molecular dynamics simulations reveal that the dimer conformation is characterized by the interaction between the hydrophobic aliphatic tails rather than the π-π stacking previously reported for other cyanine dyes. Our results pave the way for future research into the mechanism of molecular self-assembly in similar light-harvesting complexes, offering valuable insights for understanding and optimizing self-assembly processes for various (nano)technological applications.

In pick-up experiments using nanodroplet and nanocluster beams, the size distribution of hosts carrying a specified number of dopants changes when the vapor density in the pick-up region is altered. This change, analyzed here, has quantitative consequences for the interpretation of data that are sensitive to host size, such as mass spectrometric, spectroscopic, and deflection measurements.

Mechanisms that enhance charge separation at donor-acceptor interfaces are the key to material design of non-fullerene electron acceptors for high-efficiency organic photovoltaics (OPV). Here, the energetics of charge separation at the PM6-Y6 donor-acceptor interface in the state-of-the-art OPV is analyzed on the basis of quantum mechanics/molecular mechanics calculations. The electron energy level in Y6 becomes lower with increasing distance from the interface with PM6 at which the crystallinity is lower than in the bulk region. Electrostatic interactions from the multipoles of Y6 stabilize the electron in the crystalline region. The PM6-ITIC donor-acceptor interface also exhibits a similar potential cascade owing to the quadruple of ITIC. The potential cascade destabilizes charge transfer states at the PM6-Y6 interface, thereby decreasing the potential barrier for charge separation. Charge delocalization on several molecules via transfer integral further decreases the barrier for charge separation.

Machine Learning Force Fields (MLFFs) require ongoing improvement and innovation to effectively address challenges across various domains. Developing MLFF models typically involves extensive screening, tuning, and iterative testing. However, existing packages based on a single mature descriptor or model are unsuitable for this process. Therefore, we developed a package named ABFML, based on PyTorch, which aims to promote MLFF innovation by providing developers with a rapid, efficient, and user-friendly tool for constructing, screening, and validating new force field models. Moreover, by leveraging standardized module operations and cutting-edge machine learning frameworks, developers can swiftly establish models. In addition, the platform can seamlessly transition to the graphics processing unit environments, enabling accelerated calculations and large-scale parallel simulations of molecular dynamics. In contrast to traditional from-scratch approaches for MLFF development, ABFML significantly lowers the barriers to developing force field models, thereby expediting innovation and application within the MLFF development domains.

The photophysical properties of a boron dipyrromethene (Bodipy, BDP) derivative (BDP-SA) in which one F atom at the BDP core was replaced by an O atom and condensed with salicylaldehyde were investigated. This compound has a twisted molecular structure and unusually low fluorescence quantum yield (1% in toluene). No intersystem crossing was observed with a nanosecond transient absorption study. The triplet state lifetime of BDP-SA was determined to be 115 μs by photosensitizing. Femtosecond transient absorption shows a structure relaxation of ∼1.5 ps for the S1 excited state. Theoretical studies show conical intersections, which are responsible for the efficient non-radiative decay of the S1 state, which has extremely weak fluorescence.

Polyethylene glycol (PEG) is a widely used precipitant to concentrate proteins. The effect of PEG is generally understood to be an entropic attraction between proteins due to the depletion effect of PEG around proteins. However, measurements by Bloustine et al. [Phys. Rev. Lett. 96, 087803 (2006)] of the liquid-liquid phase separation (LLPS) temperature have shown that a lysozyme solution is stabilized and destabilized by the addition of low and high molecular-weight PEG, respectively. They also presented a theoretical model of the LLPS temperature as a virial expansion of the free energy and concluded that, in addition to the depletion effect, the attractive interaction between protein and PEG is necessary to explain the experiments. In the present study, theoretical calculations based on liquid-state density functional theory utilizing coarse-grained models are conducted to demonstrate that the protein-PEG effective attraction is responsible for the suppression and promotion of LLPS upon the addition of low- and high-weight PEG, respectively. In contrast, if the interactions between the protein and the PEG are solely due to the excluded volume effect, PEG of any molecular weight destabilizes the solution. These results suggest the necessity to reconsider the conventional understanding of the effects of polymer addition, which have been historically attributed to solely the depletion force.