搜索结果：The Journal of chemical physics

Amphiphilic block copolymers are a promising class of macromolecules used for creating new functional containers for delivering active agents into the cells. The diversity of these macromolecules' architectures allows for the selection of the necessary system parameters to generate the desired morphologies of these containers. Where the standard grid-search approach proves costly, optimization algorithms can directly search for the desired structure using a data-aware approach. The challenge here lies in the numerical characterization of such vesicles with a single objective function, that is, finding a functional capable of indicating the degree of proximity of the aggregate to the desired class of structures, for example, vesicles. This paper describes a novel pipeline for the topology-directed search for molecular parameters that enable the spontaneous formation of spherical vesicles in a system of amphiphilic comb-coil copolymers with variable architectural and solvent parameters within simulations. A key feature of the presented method is the use of a probabilistic classification model, trained on ideal structures, as a functional describing the "vesicularity" of the aggregate obtained in the point-based simulation. The classification of structures is based on topological data analysis, and, as shown, topological characteristics are sufficient to accurately distinguish typical structures self-assembling in solutions of amphiphilic macromolecules. Optimization of this probabilistic functional using a classic Bayesian optimization algorithm allows finding parameters consistent with spherical vesicle formation in a reasonable number of iterations. By design, the usage of this methodology is not restricted by the considered class of polymers and can be applied to other simulated macromolecular systems.

Electrophoretic deposition is a method of choice for generating coatings thanks to its ease of implementation and its ability to produce coatings of relatively large thicknesses in a single-step process. While this process also benefits from a large number of tunable parameters to adapt the coating to each application (such as applied electric field, particle concentration, and viscosity of the suspension), such freedom can make selecting parameters an overwhelming task. A better fundamental understanding of the microscopic phenomena and mechanisms at play during deposition can provide clues for a more efficient design of optimized coatings. Particle-based models, which allow for the systematic simulation of deposit microstructures across various process parameters, are particularly interesting for gaining insights into such systems. Nevertheless, such studies are rare and usually do not include the possibility of self-cohesion between particles, which is crucial for the final structure of the deposit. Here, we use particle-based simulations to study how barrier-limited aggregation influences the deposits formed under different applied electric fields. We show that self-cohesion indeed leads to different microstructures, both in the close vicinity of the substrate and in the bulk of the deposit, and we relate this to the mechanical signature of the deposits. Our results reveal that at high electric fields, the influence of self-cohesion on the resulting microstructures essentially vanishes beyond a critical field strength. This marks the transition from a deposition regime affected by aggregation to a regime largely dominated by volume-exclusion effects.

The quantum embedding approach offers an efficient way for large-scale electronic-structure calculations, enabling reductions in computational cost while retaining high-level accuracy. This aspect is particularly crucial for the modeling of actinide solution systems. Here, we present a subsystem approach for predicting valence electron binding energies, including the electron affinity (EA) and the ionization potential (IP) of the solvated uranyl. This approach combines the equation-of-motion coupled cluster method for electron attachment/detachment and the mean-filed treatment via absolutely localized projection-based embedding scheme, using representative structures extracted from ab initio molecular dynamics simulations, together with our newly developed norm-conserving actinide pseudopotentials. Our results show that the EA of solvated uranyl can be well reproduced by using the quantum embedding approach, whereas the IP requires an expanded active high-level region because of its global character. Including the full first solvation shell yields a marked improvement for the IP. This work demonstrates that the accuracy of the embedding partition is determined not only by the apparent structural locality of the system but also by how spatially localized the electronic-structure property of interest is. We anticipate that the quantum embedding approach will provide an efficient route to reducing the computational cost of high-level quantum chemical calculations for complex actinide solution systems.

We investigated the energetics and bonding of lithium hydride clusters (LiH)n (n = 1-6) using a composite ab initio scheme inspired by W2 theory to achieve sub-kcal/mol accuracy. This approach combines CCSD(T) results extrapolated to the complete basis set limit with a rigorous treatment of core-valence correlation, scalar relativistic effects, and the diagonal Born-Oppenheimer correction. Our results show that while Hartree-Fock theory captures the primary electrostatic binding, correlation effects are crucial for determining the energetic preference of compact isomers over cyclic rings. A parallel density functional theory study shows that while standard hybrid functionals like B3LYP-D4 and M06-2X exhibit larger deviations, the double-hybrid revDSD-PBEP86-D4 functional closely matches our benchmarks, delivering sub-kcal/mol accuracy. Structural and chemical bonding analyses, including intrinsic bond orbital, nucleus-independent chemical shift, and many-body expansion (MBE) methods, reveal high ionic character and multi-center bonding (3c-2e and 4c-2e) within the (LiH)n clusters. MBE analysis of the interaction energy of the monocyclic clusters with respect to the LiH molecules reveals that the two- and three-body terms are consistently negative (stabilizing), while all higher-order terms are negligible. We find that σ-aromaticity in these systems is predominantly local and bond-centered. As the rings expand, the interior becomes magnetically decoupled from the σ-skeleton, precluding the formation of a global ring current. These results establish definitive benchmarks for the stability of prototypical electron-deficient clusters.

In this work, we present a compact analytical approximation for the quantum partition function of systems composed of quantum oscillators. The proposed formula is general and applicable to an arbitrary number of oscillators described by a rather general class of potential energy functions (not necessarily polynomials). Starting from the exact path-integral expression of the partition function, we introduce a temperature-dependent Gaussian approximation for the high-temperature propagator and then invoke a principle of minimal sensitivity to minimize the error. This leads to a system of coupled nonlinear equations whose solution yields the optimal parameters of the Gaussian approximation. The resulting approximate partition function accurately reproduces thermodynamic quantities such as the free energy, average energy, and specific heat-even at zero temperature-with typical relative errors in the range of about 1%-5%. The accuracy deteriorates only moderately when the anharmonicity and coupling strengths are increased. We illustrate the performance of our analytical formula with numerical results for systems of up to ten coupled anharmonic oscillators. These results are compared with "exact" numerical results obtained via Hamiltonian diagonalization for small systems and path-integral Monte Carlo simulations for larger ones.

Quantum mechanics describes the unitary time evolution of closed systems. In practice, every quantum system interacts with the environment, leading to an irreversible loss of coherence. The spin-boson model (SBM) is central to the understanding of the fundamental process of decoherence of a two-state quantum system interacting with a bosonic heat bath, but the nature of transient dynamics in the presence of hybrid diagonal and off-diagonal system-bath interactions remains much less explored. Here, we investigate how the hybrid system-bath interactions of an Ohmic environment induce localization in the bias-free SBM. For strong coupling to the environment, localization is strongly affected by a dynamically generated bias via the renormalization of the tunneling amplitude. We find that counteracting effects of Hamiltonian parameters on non-exponential short-time dynamics and long-time population equilibration can lead to a separation of timescales and transient quantum coherent dynamics that can persist even for ultra-strong system-bath interaction. The findings offer novel insight into the equilibration behavior of quantum devices operating in the ultra-strong coupling regime.

The classification of electrolytes and prediction of their properties is a fundamental challenge in electrolyte thermodynamics. Understanding the balance of ionic interactions is key to accurately describing solution behavior. We introduce the interaction balance theory, a framework that links microscopic ionic interactions to macroscopic activity coefficients enabling the decomposition of intermolecular interactions, systematic analysis of the sources of non-ideality, and a quantitative classification of electrolytes. Application to the sodium halides NaF, NaCl, NaBr, and NaI in water and non-aqueous solvents shows that the theory captures their distinct intermolecular behaviors, distinguishing the relative contributions of long-range Coulombic forces, short-range repulsions, and solvent-mediated interactions and correlating these decompositions with the salts' solubility. Our results also show that the minimum in activity coefficient marks the end of a specific equilibrium between the cumulative short- and long-range forces, following the considered Ewald decomposition of the forces. This approach provides a clear, quantitative method to interpret experimental data, disentangle what essentially builds the non-ideality of systems, and to guide the development of thermodynamic models for electrolytes, highlighting which interactions dominate in different systems.

This work reports the infrared laser spectroscopic study of the H3+ and Hn+ (n = 5, 6, 7, 9) ions in helium droplets. The ions were produced upon electron impact ionization of helium droplets and charge transfer to small hydrogen clusters formed within the He droplets. The spectrum of H3+ shows a well-resolved rotational structure, indicating a small (∼8%) decrease in the rotational constants as compared to the gas-phase values. This small change is rationalized in terms of the small anisotropy of the H3+ interaction potential with He atoms. However, it was found that the interaction with the helium environment has a profound effect on the spectrum of the fluxional H5+ ions. Whereas the previously measured gas-phase spectrum has well-separated bands, the spectrum of H5+ in helium droplets shows a broad spectrum extending over 2000 cm-1 containing some maxima with frequencies close to those of the gas phase. The bands of H7+ and H9+ due to vibrations of their H2 moieties were also observed near 4000 cm-1. The structure of these ions could be treated as an H3+ core solvated in H2 molecules and He atoms. The spectrum of larger Hn+ clusters is also reported.

Reversible associations of RNAs among themselves, or with RNA binding proteins through the process of phase separation, are found to be important in many different cellular contexts, including cellular responses to stress, gene regulation, development, and disease. Short RNA repeat sequences, which are mostly linked with many repeat expansion neurodegenerative diseases, are found to undergo phase separation and yield protein-free biomolecular condensates in vitro and in cells. However, the physicochemical principles governing phase separation of RNAs considering both sequence and structural aspects, especially for short RNAs, remain elusive. It is intriguing, as well as challenging, to characterize the RNA phase behavior at a submolecular resolution. Based on atomistic enhanced sampling simulations, here we report potential dynamic structural effects in the mutual association properties of a tetra loop containing 14-mer hairpin RNA. We show that the folded and unfolded conformations of the hairpin fragment lead to different energetic barriers for the formation of an associated pair. Unfolded conformation leads to the formation of gel like energetically stable associated phases spontaneously; the folded hairpin motif is found to yield droplet like associated phases accompanying by cations in solution. Overall, our findings illustrate that dynamic association/dissociation is energetically more favorable for folded RNA hairpins, and the presence of additional salt in solution assists the formation of dense droplet like associated phases, while ionic concentration plays a critical role in the formation and stability of the droplet like associated phase. These observations open avenues for exploring structure-based phase separation mechanisms of RNAs in the context of RNA mediated functional biomolecular condensate formation within cells and designing RNA based novel biomaterials.

Understanding interfacial heat transfer between polymers and water is crucial for the design of biomaterials, drug delivery platforms, and nano-fluidic systems. In this study, we employed all-atom molecular dynamics (MD) simulations to quantify the interfacial thermal conductance between an infinitely diluted polyethylene glycol (PEG) 36-mer chain and explicit water over the temperature range of 280-350 K. To compare the conformational behavior of the PEG chain, we examined its radius of gyration and observed a temperature-dependent chain collapse consistent with previous coarse-grained models. By employing a transient non-equilibrium MD approach, we imposed temperature difference across the interface and analyzed the energy relaxation behavior to compute heat transfer across the polymer-water interfaces. Moreover, we investigate the impact of polymer conformation on heat transfer by considering modulations of the Lennard-Jones polymer/solvent interactions, different from the original PEG-water interactions. Our results demonstrate that both temperature and Lennard-Jones interfacial interaction strength influence interfacial thermal conductance, with temperature playing the dominant role. Structural factors such as chain conformation and interfacial area were found to mediate the effect of interfacial interaction. Additional analysis of the vibrational density of states and the mean square displacement reveal that vibrational coupling has minimal impact on thermal conductance across interfaces, whereas increased water thermal motion enhances energy transfer. These findings highlight the structural and dynamical origins of interfacial thermal conductance and provide atomistic insights into the tuning of interfacial heat transport in molecular systems through temperature and solvent interactions.

Determining the glass transition temperature Tg in materials science in general and for amorphous polymer systems in particular is a delicate matter due to the uncertainty in the definition of Tg and the complexity of the glass transition phenomenon itself. Machine learning (ML) provides powerful approaches for analyzing complex, high-dimensional data to reveal hidden patterns. Recently, we have applied an unsupervised ML technique to identify Tg of a polymer melt of weakly semiflexible bead-spring chains using the time evolvement of pairwise internal distances between monomers along chains, as input features. Here, we investigate the change of individual internal chain relaxation as the polymer melt transforms from the liquid to glassy state. The average overall relaxation remains unchanged and displays the usual temperature dependence. However, for some individual pair distances, scattered throughout the sample, relaxation is significantly delayed, which serves as a robust indicator of approaching the glass transition. Moreover, these changes and the first principle components are highly correlated. This is an evidence that the ML technique indeed captures a significant indicator of the approach of the glass transition. Typical experiments, which average over the whole sample, cannot identify such features.

We investigate the microscopic origin of collective dynamics in molecular liquids at intermediate (mesoscale) length scales using large-scale molecular dynamics simulations of tetrahydrofuran (THF). In agreement with previous neutron scattering results, in this regime, the dynamic structure factor S(Q, t) of THF exhibits-apart from collective excitations-a non-diffusive relaxation process (the Q0-mode) characterized by a relaxation time that is nearly independent of wavevector transfer Q. Our results confirm that this behavior arises from a cancellation of diffusive contributions between the self and distinct components of S(Q, t), extending previous findings for water to a non-hydrogen bonded system with moderate dipolar interaction. By systematically comparing coherent and incoherent scattering functions, we demonstrate that the Q0-mode originates from localized center-of-mass motions occurring within transient cages formed by neighboring molecules. These motions are directly linked to the caged regime of the mean squared displacement and define a characteristic time and length scale for the onset of Fickian diffusion. In contrast, the corresponding relaxation observed in the incoherent scattering function of hydrogen atoms FsH(Q,t) includes additional contributions from molecular rotations, leading to longer relaxation times for the fast localized process. A detailed analysis of rotational dynamics shows that translational and rotational motions are weakly coupled and can be approximately factorized, allowing a quantitative interpretation of the differences between coherent and incoherent responses. The resulting picture provides a unified microscopic framework for mesoscale dynamics in molecular liquids and rationalizes recent neutron scattering experiments.

At high operating voltages, high-voltage spinel LiNi0.5Mn1.5O4 (LNMO) undergoes serious lattice distortion and Jahn-Teller effect, accompanied by slow Li+ diffusion kinetics, which greatly hinders its practical deployment. Herein, Al-substituted LNMO (LiNi0.5Mn1.5-xAlxO4) was prepared by ion infiltration, and the influences of Al doping on structural and electrochemical performances were comprehensively explored. Combined physicochemical characterizations verify that Al3+ preferentially occupies the 16d crystallographic sites of the spinel lattice. This modification reduces cation mixing and induces a mild lattice contraction, with no secondary phases detected. Electrochemical measurements demonstrate that the optimized LNMO-A0.05 possesses superior rate performance and cyclability. After 100 cycles at 0.2C, it maintains capacity retentions of 95.1% (25 °C) and 89.9% (55 °C). In situ XRD analyses confirm that Al doping effectively suppresses lattice distortion and irreversible phase evolution, switching the electrochemical reaction from a two-phase mechanism to a solid-solution reaction. Electrochemical impedance spectroscopy (EIS) characterizations reveal that Al doping reduces charge transfer resistance and, accordingly, elevates the Li+ diffusion coefficient to 7.91 × 10-16 cm2/s. Further density functional theory calculations demonstrate that the robust Al-O covalent bonds function as structural anchors to reinforce the lattice. Meanwhile, electronic delocalization accelerates ion migration and improves reaction kinetics. The assembled LNMO-A0.05/LTO full cell delivers remarkable performance, demonstrating great potential for practical use.

We present ab initio calculations of total and orbital-resolved photoionization cross sections for the C2H4O2 isomers glycolaldehyde, acetic acid, and methyl formate from threshold up to 40 eV. The calculations were performed using the ePolyScat framework at the static-exchange (SE) and SE-plus-polarization levels. The total photoionization cross sections, obtained as the sum over all twelve valence orbitals, exhibit a similar overall energy dependence for all isomers, with systematic differences near the ionization threshold and in the region of the cross section maximum. In contrast, the orbital-resolved analysis reveals a pronounced isomer dependence. To justify the single-particle picture underlying the scattering calculations, electron propagator theory was employed; the calculated pole strengths for the outer-valence orbitals remain high (∼0.9), validating the use of Hartree-Fock initial states as adequate representations of the Dyson orbitals. The highest occupied molecular orbitals, dominated by nonbonding oxygen lone pairs, give rise to the largest cross sections. Pronounced shape resonances are observed in the HOMO channels in the 15.5-19.3 eV range, with positions and intensities strongly dependent on molecular structure. Partial-wave analysis shows that these features are primarily governed by the l = 4 component, with additional contributions from neighboring channels. These results demonstrate that, despite identical stoichiometry, differences in molecular structure lead to distinct continuum electron dynamics. The computed cross sections provide quantitative input for astrochemical models and provide insights into isomer-dependent behavior in UV-irradiated environments.

We present q-vSZPs, a polarized, environment-adaptive minimal Gaussian basis set parameterized for elements Z = 1-86 (excluding lanthanides) and designed for efficient mean-field quantum-mechanical calculations on large molecular systems. Building on the previously introduced q-vSZP basis set, the number of primitive functions is substantially reduced while retaining the key feature of charge- and coordination-number-dependent contraction coefficients, which enable environment-dependent "breathing" of the atomic orbitals. The effective atomic charges required for the basis set setup are obtained from the bond-capacity electronegativity equilibration model (EEQBC), which accurately reproduces density functional theory (DFT)-level Hirshfeld charges at negligible computational cost. Comprehensive benchmarking on the GMTKN55 database demonstrates that q-vSZPs incurs only a moderate loss in accuracy relative to q-vSZP (WTMAD-2 increase of 2.9 kcal⋅mol-1), while substantially outperforming conventional non-adaptive minimal basis sets (MBs). For non-covalent interactions, the performance even surpasses that of the double-ζ def2-SVP basis set, owing to a substantially lower basis set superposition error as a consequence of the molecular optimization strategy. A non-adaptive variant using averaged charges and coordination numbers (qavg-vSZPs) is additionally provided as a drop-in minimal basis for any quantum-chemical program. For a representative 4108-atom system, q-vSZPs achieves a twofold reduction in both SCF wall time and memory consumption for DFT calculations relative to q-vSZP, placing it in the efficiency range of other MBs. The utility of q-vSZPs is further demonstrated through a mixed-basis application for the computation of the Raman spectrum of adenine in water.

Accurate modeling of bond breaking remains a central challenge for reduced density matrix functional theory (RDMFT). Although some modern functionals can yield reasonably accurate dissociation energies, they often fail to reproduce key properties of the dissociated fragments, such as a vanishing fragment population covariance (also known as the delocalization index) and the correct total spin angular momentum of each fragment (local spin). In this study, we revisit the construction of natural orbital functionals by correcting the cumulant contribution produced by the PNOF5 functional. Our method enforces known contributions of the cumulant to local spin fragments and the delocalization index at the dissociation limit. We obtain the closest cumulant consistent with these physically motivated constraints and subsequently purify the corresponding one- and two-electron reduced density matrices by imposing the standard P, Q, and GN-representability conditions. The resulting functional yields improved behavior in strongly correlated regimes. Benchmarking on the dissociation of the singlet states of N2, NO+, O2, S2, and CO shows that, in the dissociation regime, the energies computed from the updated cumulant exactly reproduce the complete active space self-consistent field energies. We further analyze the limitations of the approach and identify scenarios in which the current approach performs poorly. This study provides a pathway for systematically improving natural orbital functionals to achieve reliable bond-breaking calculations within RDMFT.

In nuclear magnetic resonance imaging, superparamagnetic iron oxide nanoparticles (SPIONs) are an alternative contrast agent to gadolinium, as they can be used as negative contrast agents. They are usually coated with polymers or sugars for stability and biocompatibility. Coatings are at best semipermeable to water. In existing theories of SPION-induced nuclear magnetic resonance contrast, T2 contrast relies on diffusion of water protons in the magnetic field inhomogeneities created by the nanoparticles; therefore, reduced diffusion in the coating is expected to impact the relaxation rate of protons surrounding SPIONs. In this study, the impact on transverse relaxation of a coating layer surrounding SPIONs in which water diffusion is slowed is studied through a Monte Carlo algorithm. It is shown that for small SPIONs, the presence of a coating does affect the transverse relaxation rate R2. The transverse relaxation rate is higher and, therefore, SPION-induced contrast performance is enhanced for nanoparticles with a magnetic iron oxide core smaller than 10 nm in radius, with sufficiently thick coatings with water diffusion coefficients typical of polyethylene glycol, dextran, and gelatin. Those R2 variations can be semi-quantitatively understood using an exchange model, where the protons inside the coating contribute to the relaxation rate with a weight that takes into account their residence time in the coating. Eventually, this study provides guidelines for SPION synthesis for optimum contrast performance: small SPIONs with bigger coatings where the diffusion coefficient of water is 3-10 times lower than the self-diffusion coefficient of water perform the best as T2 contrast agents. Such coatings can be polyethylene glycol or polyacrylic acid, dextran, and gelatin.

The transfer tensor method (TTM) is a versatile tool for analyzing and propagating general open quantum systems. It captures in a compact manner all memory effects in a non-Markovian system through a straightforward transformation of a set of dynamical maps. Transfer tensors (TTs) provide the exact convolutional propagator associated with a given time discretization over the past evolution of an open quantum system. Here we show that, for any finite time discretization, the memory kernel of the Nakajima-Zwanzig equation deviates from the exact TTs, although both converge in the continuous-time limit, as expected. We examine this behavior in the context of an analytically solvable model: a two-level atom resonant with a lossy cavity in the Jaynes-Cummings limit. The atomic dynamics separate into two decoupled subspaces-the coherence and the population difference. We derive exact expressions for the dynamical map, the TTs, and the memory kernel governing the coherence, and we relate them to their counterparts for the population difference. As a function of the ratio between the cavity loss rate and the atom-cavity coupling strength, we identify regions of enhanced non-Markovianity in which the dynamics can be described as fully Markovian for certain TTM time-step choices.

High-level ab initio estimates for the equilibrium structure (rCC = 1.330 79 ± 0.000 31 Å, rCH = 1.080 75 ± 0.000 21 Å, and αCCH = 121.423° ± 0.023°), the adiabatic ionization energy (10.52 eV), and the 0 K proton affinity (673.2 kJ/mol) of ethylene were obtained via focal-point extrapolation of coupled cluster results to the basis set limit. This methodology also provided accurate equilibrium structures for the ethylene radical cation (ionized ethylene) and the ethenium cation (bridged protonated ethylene). An additive correction scheme allowed for the prediction of the vibrational spectrum of ethylene with a root-mean-square error of 2.4 cm-1. A systematic evaluation of popular wavefunction-based methods revealed that the recently implemented orbital-optimized linearized coupled cluster (OLCCD) method predicts the geometry and properties (the ionization potential, the proton affinity, and the fundamental frequencies) of ethylene significantly better than the orbital-optimized Møller-Plesset (OMP2 and OMP3) and the standard coupled cluster with singles and doubles methods. Notably, where coupled-cluster with single, double, and perturbative triple excitations analytic Hessians suffer from wavefunction instability issues along several normal modes, OLCCD yields superior vibrational anharmonicities, making it a robust alternative for challenging π-systems.