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Revealing the process-structure-property (PSP) relationships of chemically complex mixed-ion perovskite requires comprehensive insights into correlations between microstructures and chemical compositions. However, experimentally determining the microstructural information about complex perovskites over the composition space is a challenging task. In this study, a machine learning enabled energy model was trained for MAyFA1-yPb(BrxI1-x)3 mixed-ion perovskite for fast and extensive sampling over the compositional/permutational spaces to map the ion-mixing energies, chemical ordering, and atomic strains. Correlation analysis indicated the strong lattice distortion in the high-MA/Br concentration regime is the primary reason for poor device performance-strong lattice distortion induces high mixing energy, resulting in phase segregation and defect formation. Hence, mitigating lattice distortion to retain the single-phase solid solution is one necessary condition of the optimal composition of mixed-ion perovskites. The present study therefore provides insights into the microstructures as well as the guidelines for determining the optimal composition of mixed-ion perovskite materials.

The optical Kerr effect (OKE) spectroscopy measured with heterodyne detection (HD) is a useful tool to provide information regarding intermolecular vibrations and structural relaxations in liquid water. Recently, the measurements of the OKE spectroscopy have been extended to the supercooled regime of water. Though the measured results can be well described by using a phenomenological model, the time-resolved OKE spectroscopy of liquid and supercooled water still need a comprehensive understanding. In this paper, we investigated the OKE nuclear response functions of this peculiar liquid and their reduced spectral densities by performing molecular dynamics simulations with the TIP4P/2005 water model. The collective polarizability of water was computed via a dipolar induction scheme, which involves the intrinsic polarizability and the first-order hyperpolarizability tensor of water molecule. Our simulation results were qualitatively consistent with the HD-OKE experimental observations for displaying that the polarizability anisotropy relaxation of supercooled water in the high-density liquid phase was fragile-like by following a stretching exponential decay with an exponent βs insensitive to temperature and the temperature dependence of the relaxation time exhibited a power-law divergence at a singular temperature Ts with a critical exponent γs. Indicated by our quantitative results, Ts was predominately determined by the structural arrest, but βs and γs were not only related to the structural relaxation but also influenced by the collective polarizability of the liquid. For all investigations, the effects due to the first-order hyperpolarizability tensor were examined.

The OH-group connecting bifurcated H-bond (HB) acceptors is one case of the three-centered HBs found in physical and biological systems. An example is the transition state of molecular reorientation jump mechanism in liquid water, ionic solutions, and hydration shells of biomolecules. In this Letter, the orientational time correlation functions of these special OH-groups in water were studied by performing molecular dynamics simulations and using the normal-mode approach developed recently. The reorientation angle of the OH-group before entering the diffusive regime can be estimated through the plateau in the mean-square rotational displacement calculated by several methods.

The correlation between the optical Kerr effect (OKE) spectroscopy of supercooled water at ambient pressure and its structural tetrahedrality was investigated by contrasting simulation results of two non-polarizable water models modified with the same collective polarizability, which involves intrinsic molecular polarizability and induced polarizability arising from interactions between molecular dipoles. The tetrahedrality of water structure was typified with the second-peak maximum in the pair distribution function of oxygens and the fraction of molecules, which and their neighbours up to the second hydration shell all have four H-bond coordinators. Our results indicate that the intermolecular vibrational band in the OKE spectrum of supercooled water is considerably correlated to its structural tetrahedrality.

The mesoscale morphologies of organic small molecular films fabricated via vacuum deposition processes are critical to the performance of small-molecule solar cells and organic light-emitting diodes. In the present study, the morphological evolution of the active layer of DPDCPB:${\mathrm{C}}_{70}$ small-molecule solar cells during vacuum codeposition processes was revealed by a series of GPU-accelerated coarse-grained molecular-dynamics simulations. The ${\mathrm{C}}_{70}$ and DPDCPB molecules were coarsened into ellipsoids and bonded ellipsoids, respectively. The interactions between ellipsoids were described by the Gay-Berne formulation and were parametrized to reproduce potential energy surfaces from all-atom atomistic simulations using a genetic algorithm. Due to the significantly reduced overall degrees of freedom, this coarse-grained scheme allowed us to simulate the vacuum codeposition processes and monitor the morphological evolution of systems with system length scales compatible with those of the experiments (\ensuremath{\sim}30 nm). Our simulations indicate that the film morphologies are closely correlated with the DPDCPB:${\mathrm{C}}_{70}$ blending ratio. High ${\mathrm{C}}_{70}$ concentration leads to a rough film surface, which is in accordance with experimental observations and can be attributed to the strong self-aggregation behavior of ${\mathrm{C}}_{70}$ molecules. The morphological property analysis indicates that the rough film surface has an almost negligible impact on the DPDCPB/${\mathrm{C}}_{70}$ domain percolations, and the device with the optimal deposition ratio should give the most balanced hole/electron transfer in respective DPDCPB/${\mathrm{C}}_{70}$ domains. The present study demonstrates that by using the ellipsoid-based coarse-grained model, it is possible to study the morphological evolution of small-molecule organic thin film during vacuum deposition processes with molecular scale details, which can provide valuable insights for experimental teams to further optimize device fabrication protocols for the next generation of organic optoelectronic devices.

In this paper, we investigated the tetrahedral structure of supercooled water at ambient pressure and its influence on dynamic relaxation by comparing simulation results of TIP4P/2005 and SPC/E water models. The globally tetrahedral structure of supercooled water was characterized with the second-peak maximum and a deep first minimum in the radial distribution function g ( r ) of O-atoms and the reverse order in magnitude between the first two peaks of structure factor. The locally tetrahedral structure was specified by molecules, which and their neighbors up to the second hydration shell all have four H-bond coordinators. These molecules are referred as low-density liquid (LDL) and the others as high-density liquid (HDL). The water dynamics relaxation was studied through the self-intermediate scattering function, the non-Gaussian parameter, and the polarizability anisotropy time correlation function. Indicated by our simulations, the temperature dependence of the stretched exponent describing the α -relaxation displayed a small peak in the supercooled regime above the Widom line (WL), where LDL at the peak temperature was roughly one fourth of the total. The stretched exponent depicting the polarizability anisotropy relaxation was found to be insensitive to temperature, consistent with the experimental results. Above the WL, all relaxation times studied displayed a power-law temperature dependence with a consistent singular temperature for each model. The inverse relaxation times showed exponential functions of two-body excess entropy due to translational motions, where the entropy exhibited a logarithmic temperature behavior with a singular temperature close to that of relaxation time. This result leads to a conclusion that excess entropy is a quantity to describe dynamic relaxation of supercooled water in the thermodynamic region where the mode-coupling theory works. The water structure that causes the two-body excess entropy is illustrated and the contributions of HDL, LDL, and their mixing are also shown, where the mixing contributes significantly as near the WL. Below the WL, the formula based on the two-body excess entropy may no longer be valid.

In this paper, we investigated the optical Kerr effect (OKE) reduced spectral density (RSD) of liquid and supercooled water at ambient pressure and focused on the contributions of low-density liquid (LDL) and high-density liquid (HDL), which local structures are characterized with tetrahedral order and disorder, respectively. The water systems were generated by performing molecular dynamics (MD) simulations with TIP4P/2005 force fields, and molecules were classified into LDL and HDL with the microscopic structural descriptor [Nature commun. 3 (2014) 3556.]. The OKE RSD was calculated with a model involving the intrinsic polarizabilities of individual molecules and the dipole-induced-dipole mechanism between them. The instantaneous normal mode (INM) analysis was used to dissect the OKE RSD obtained by MD simulations. By considering water as a mixture of LDL and HDL, the OKE RSD in the linearized INM theory is a sum of three contributions, which are resulted from the two partial liquids individually and a cross correlation associated with both of them, in a formulism similar as that for the static structure factor of a binary mixture. By exploring with two separation schemes on the collective polarizability, where they are complemental to each other, the INM analysis provides an insight into the physical causes for the bands in the OKE RSD of liquid and supercooled water and the relation of each band to the local structures of LDL and HDL.

In terms of the local bond-orientational order (LBOO) parameters, a cluster approach to analyze local structures of simple liquids was developed. In this approach, a cluster is defined as a combination of neighboring seeds having at least nb local-orientational bonds and their nearest neighbors, and a cluster ensemble is a collection of clusters with a specified nb and number of seeds ns. This cluster analysis was applied to investigate the microscopic structures of liquid Ga at ambient pressure (AP). The liquid structures studied were generated through ab initio molecular dynamics simulations. By scrutinizing the static structure factors (SSFs) of cluster ensembles with different combinations of nb and ns, we found that liquid Ga at AP contained two types of cluster structures, one characterized by sixfold orientational symmetry and the other showing fourfold orientational symmetry. The SSFs of cluster structures with sixfold orientational symmetry were akin to the SSF of a hard-sphere fluid. On the contrary, the SSFs of cluster structures showing fourfold orientational symmetry behaved similarly as the anomalous SSF of liquid Ga at AP, which is well known for exhibiting a high-q shoulder. The local structures of a highly LBOO cluster whose SSF displayed a high-q shoulder were found to be more similar to the structure of β-Ga than those of other solid phases of Ga. More generally, the cluster structures showing fourfold orientational symmetry have an inclination to resemble more to β-Ga.

By simulating the rigid simple point charge extended model at temperature T = 300 K, the orientational relaxation of the OH-bond in water was investigated over short to intermediate timescales, within which molecules undergo inertial rotation and libration and then enter the rotational diffusion regime. According to the second-cumulant approximation, the orientational time correlation function (TCF) of each axis that is parallel or perpendicular to an OH-bond is related to an effective rotational density of states (DOS), which is determined using the power spectra of angular velocity autocorrelation functions (AVAFs) of the other two axes. In addition, the AVAF power spectrum of an axis was approximated as the rotational stable instantaneous normal mode (INM) spectrum of the axis. As described in a previous study [S. L. Chang, T. M. Wu, and C. Y. Mou, J. Chem. Phys. 121, 3605 (2004)], simulated molecules were classified into subensembles, according to either the local structures or the H-bond configurations of the molecules. For global molecules and the classified subensembles, the simulation results for the first- and second-rank orientational TCFs were compared with the second-cumulant predictions obtained using the effective rotational DOSs and the rotational stable-INM spectra. On short timescales, the OH-bond in water behaves similar to an inertial rotor and its anisotropy is lower than that of a water molecule. For molecules with three or more H-bonds, the OH-bond orientational TCFs are characterized by a recurrence, which is an indication for libration of the OH-bond. The recurrence can generally be described by the second-cumulant prediction obtained using the rotational stable-INM spectra; however, the orientational TCFs after the recurrence switch to a behavior similar to that predicted using the AVAF power spectra. By contrast, the OH-bond orientational TCFs of molecules initially connected with one or two H-bonds decay monotonically or exhibit a weak recurrence, indicating rapid relaxation into the rotational diffusion regime after the initial Gaussian decay. In addition to accurately describing the Gaussian decay, the second-cumulant predictions formulated using the rotational stable-INM spectra and the AVAF power spectra serve as the upper and lower limits, respectively, for the OH-bond orientational TCFs of these molecules after the Gaussian decay.

By exploiting the instantaneous normal mode (INM) analysis for models of flexible molecules, we investigate intermolecular and intramolecular vibrations of water from the atomic point of view. With two flexible SPC/E models, our investigations include three aspects about their INM spectra, which are separated into the unstable, intermolecular, bending, and stretching bands. First, the O- and H-atom contributions in the four INM bands are calculated and their stable INM spectra are compared with the power spectra of the atomic velocity autocorrelation functions. The unstable and intermolecular bands of the flexible models are also compared with those of the SPC/E model of rigid molecules. Second, we formulate the inverse participation ratio (IPR) of the INMs, respectively, for the O- and H-atom and molecule. With the IPRs, the numbers of the three species participated in the INMs are estimated so that the localization characters of the INMs in each band are studied. Further, by the ratio of the IPR of the H atom to that of the O atom, we explore the number of involved OH bond per molecule participated in the INMs. Third, by classifying simulated molecules into subensembles according to the geometry of their local environments or their H-bond configurations, we examine the local-structure effects on the bending and stretching INM bands. All of our results are verified to be insensible to the definition of H-bond. Our conclusions about the intermolecular and intramolecular vibrations in water are given.

This paper studies the melting behavior of Ag14 cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag17Cu2. The isothermal Brownian-type molecular dynamics simulation is used to generate atom configurations of Ag14 at different temperatures up to 1500 K. At each temperature, these atomic configurations are then analyzed by the INM technique. To delve into the melting behavior of Ag14 cluster which differs from Ag17Cu2 by the occurrence of an anomalous prepeak in the specific heat curve in addition to the typical principal peak, we appeal to examining the order parameter τ(T) defined in the context of the INM method. Two general approaches are proposed to calculate τ(T). In one, τ(T) is defined in terms of the INM vibrational density of states; in another, τ(T) is defined considering the cluster as a rigid body with its rotational motions described by three orthogonal eigenvectors. Our results for Ag14 by these two methods indicate the mutual agreement of τ(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter τ(T) provides in addition an insightful interpretation between the melting of clusters and the concept of broken symmetry which has been found successful in studies of the melting transition of bulk systems. © 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4772096]

We perform isothermal Brownian-type molecular dynamics simulations to obtain the velocity autocorrelation function and its time Fourier-transformed power spectral density for the metallic cluster Ag17Cu2. The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature Tm, which we define to be the principal maximum position of the specific heat is determined. The instantaneous normal mode analysis is then used to dissect the cluster dynamics by calculating the vibrational instantaneous normal mode density of states and hence its frequency integrated value Ij which is an ensemble average of all vibrational projection operators for the jth atom in the cluster. In addition to comparing the results with simulation data, we look more closely at the entities Ij of all atoms using the point group symmetry and diagnose their temperature variations. We find that Ij exhibit features that may be used to deduce Tm, which turns out to agree very well with those inferred from the power spectral density and specific heat. © 2011 American Institute of Physics. [doi:10.1063/1.3628669] I. INTRODUCTION Given an n-body empirical potential, one can show 1 that a 13-atom silver cluster has an icosahedral geometry in its lowest energy state. This icosahedral structure is unaffected even if we replace one Ag atom by a Cu atom. The cluster’s lowest energy structure remains an icosahedron but in this case the Cu atom occupies the central position of the icosahedron. If one considers a seven-atom cluster comprising six Ag atoms and one Cu atom, its lowest energy structure is a pentagonal bipyramid with the Cu atom residing at one of the apex positions. Interestingly, a 19-atom cluster consisting of 2 Cu atoms and 17 Ag atoms takes on its ground state geometry by simply joining the 7- and 13-atom clusters in which the Cu atom in the 7-atom cluster substitutes one of the apex atoms in the 13-atom cluster thus forming a double icosahedron. The 19-atom bimetallic cluster (BC), Ag17Cu2, is therefore characterized by two “center” atoms Cu locating inside 17 “surface” atoms Ag and the atoms assume a geometry of a double icosahedron. This lowest energy structure is consistent with our understanding of a BC that an atom of smaller size (ionic radii of Cu and Ag are 0.96 A and 1.26 A, respectively) prefers to be surrounded by atoms of larger size. This mixing tendency arises from the size-mismatched disparity and has previously been noted in the literature for the noble-metalbased BCs. 1‐3