P. J. Hsu

Structures of metallic clusters: Mono- and polyvalent metals

We present detailed numerical results on the ground state structures of metallic clusters. The Gupta-type many-body potential is used to account for the interactions between atoms in the cluster. Both the genetic algorithm technique and the basin hopping method have been applied to search for the global energy minima of clusters. The excellent agreement found in both schemes for the global energy minima gives credence to the optimized energy values obtained. For four monovalent and one polyvalent metals studied in this work and within the accuracy of the energies presented here, we find that the global energy minima predicted by the basin hopping method are the same as those values obtained by the genetic algorithm. Our calculations for the ground state energies of alkali metallic clusters show regularities in the energy differences, and the cluster growth pattern manifested by this same group of clusters is generally icosahedral, which is quite different from the close-packed and decahedral preferentiall...

Structures of bimetallic clusters.

We report an optimization algorithm for studying bimetallic nanoclusters. The algorithm combines two state-of-the-art methods, the genetic algorithm and the basin hopping approach, widely employed in the literature for predicting structures of pure metallic and nonmetallic clusters. To critically test the present algorithm and its use in determining the lowest-energy structures of bimetallic nanoclusters, we apply it to study the bimetallic clusters Cu(n)Au(38-n) (0< or =n< or =38). It is predicted that the Au atoms, being larger in size than the Cu atoms, prefer to occupy surface sites showing thus the segregating behavior. As the atom fraction of Cu increases, the bimetallic cluster Cu(n)Au(38-n), as a whole, first takes on an amorphous structure and is followed by dramatic changes in structure with the Cu atoms revealing hexagonal, then assuming pentagonal, and finally shifting to octahedral symmetry in the Cu-rich range.

Multicanonical basin hopping: A new global optimization method for complex systems

We introduce a new optimization algorithm that combines the basin-hopping method, which can be used to efficiently map out an energy landscape associated with minima, with the multicanonical Monte Carlo method, which encourages the system to move out of energy traps during the computation. As an example of implementing the algorithm for the global minimization of a multivariable system, we consider the Lennard-Jones systems containing 150-185 particles, and find that the new algorithm is more efficient than the original basin-hopping method.

Melting scenario in metallic clusters

The isothermal Brownian-type molecular dynamics simulation has been applied to study the melting behavior of bimetallic clusters. It was found that the specific heat and Lindermann-like parameter customarily used in bulk system to describe solid-liquid transition show incongruity in the predicted melting temperature T(melt). The underlying mechanisms that lead to the incompatibility of T(melt) separately deduced from these two quantities were analyzed further. To gain insight into the melting behavior, we calculated in addition the velocity autocorrelation function and its Fourier transform, the power spectrum, and extracted from them the T(melt). It appears that the T(melt) inferred from the latter quantities is closer to that deduced from the principal peak position of specific heat. Two bimetallic clusters, namely, Ag(1)Cu(13) and Au(1)Cu(13), were selected for a thorough investigation. In the context of cluster morphology, we scrutinized the atomic distributions of Ag(1)Cu(13), Au(1)Cu(13), and Cu(14) and effected a comparative study between a bimetallic cluster and a pure cluster so as to learn from comparison the differences in the thermal reaction of atoms, in particular, the impurity atom in the bimetallic cluster. On analyzing the dynamical data, we observed at a lower temperature (T<<T(melt)) migrational relocation of atoms whose dynamics was superimposed at an intermediate temperature (T

Comparative study of cluster Ag17Cu2 by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation

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 Ag(17)Cu(2). The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature T(m), 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 I(j) 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 I(j) of all atoms using the point group symmetry and diagnose their temperature variations. We find that I(j) exhibit features that may be used to deduce T(m), which turns out to agree very well with those inferred from the power spectral density and specific heat.

Dependence on size of supported Rh nanoclusters for CO adsorption

We studied the adsorption and lateral interactions of CO molecules on Rh nanoclusters supported on an ordered thin film of Al2O3/NiAl(100) with varied surface probe techniques under ultra-high vacuum conditions and with density-functional-theory (DFT) calculations. The Rh clusters were grown with vapor deposition onto the Al2O3/NiAl(100) surface at 300 K; with increasing deposition, their mean diameter evolved from 1.0 to 3.5 nm and their height from 0.4 to 0.8 nm. The initial adsorption energy (for sparse CO coverage) and the number of adsorbed CO per surface site on the Rh clusters increased with decreasing cluster size. The former effect results from the surface structure and expanded lattice parameter of small Rh clusters, whereas the latter effect involves not only the initial adsorption energy but also altered lateral interactions among CO molecules. In contrast with CO on Rh single crystals, CO on small clusters adsorbed with their axes tilted from the surface normal, weakening the CO–CO repulsive interactions for CO coverage over a wide range. The saturated density of CO on clusters of diameter near 1.0 nm and height near 0.4 nm is 2–3 times that of large clusters (diameter ≥ 3.5 nm) or a Rh(100) surface. The CO–CO repulsive interactions on small clusters became effective at large CO densities, given that the onset of desorption of CO at saturation was 100 K lower than that of large clusters.

Melting behavior of Ag14 cluster: An order parameter by instantaneous normal modes

This paper studies the melting behavior of Ag(14) cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag(17)Cu(2). The isothermal Brownian-type molecular dynamics simulation is used to generate atom configurations of Ag(14) 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 Ag(14) cluster which differs from Ag(17)Cu(2) 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 Ag(14) 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.

Precursory signatures of protein folding/unfolding: from time series correlation analysis to atomistic mechanisms.

Folded conformations of proteins in thermodynamically stable states have long lifetimes. Before it folds into a stable conformation, or after unfolding from a stable conformation, the protein will generally stray from one random conformation to another leading thus to rapid fluctuations. Brief structural changes therefore occur before folding and unfolding events. These short-lived movements are easily overlooked in studies of folding/unfolding for they represent momentary excursions of the protein to explore conformations in the neighborhood of the stable conformation. The present study looks for precursory signatures of protein folding/unfolding within these rapid fluctuations through a combination of three techniques: (1) ultrafast shape recognition, (2) time series segmentation, and (3) time series correlation analysis. The first procedure measures the differences between statistical distance distributions of atoms in different conformations by calculating shape similarity indices from molecular dynamics simulation trajectories. The second procedure is used to discover the times at which the protein makes transitions from one conformation to another. Finally, we employ the third technique to exploit spatial fingerprints of the stable conformations; this procedure is to map out the sequences of changes preceding the actual folding and unfolding events, since strongly correlated atoms in different conformations are different due to bond and steric constraints. The aforementioned high-frequency fluctuations are therefore characterized by distinct correlational and structural changes that are associated with rate-limiting precursors that translate into brief segments. Guided by these technical procedures, we choose a model system, a fragment of the protein transthyretin, for identifying in this system not only the precursory signatures of transitions associated with α helix and β hairpin, but also the important role played by weaker correlations in such protein folding dynamics.

The interaction of CO molecules on Au–Rh bimetallic nanoclusters supported on a thin film of Al2O3/NiAl(100)

The interaction of CO molecules adsorbed on Au–Rh bimetallic nanoclusters supported on an ordered thin film of Al2O3/NiAl(100) was studied, primarily with infrared reflection absorption spectroscopy and density-functional-theory calculations. The bimetallic clusters, grown by sequential deposition of vapor Au and Rh onto the Al2O3/NiAl(100) surface at 300 K, had diameters of 1.2–3.0 nm and heights of 0.4–1.2 nm; they had a fcc phase and grew in the orientation (100). The infrared absorption line for CO adsorbed on Au sites (COAu) of the bimetallic clusters at 110 K was narrow (centered about 2100 cm−1) and intense, which results largely from the small adsorption energy and large dipole moment of COAu, whereas that on Rh sites (CORh) was broad (1880–2100 cm−1) and weak, which contrasts also with its counterpart on pure Rh clusters. Upon increasing the temperature to remove COAu, the absorption line for CORh narrowed and the intensity increased; at 300 K, the line width decreased by 30–40% and the absorption intensity was enhanced by 40–60%. The former arose, after the desorption of COAu, from a decreased CO–CO interaction and inhomogeneous broadening; the latter corresponded to an enhanced dipole moment of CORh, attributed to a promoted charge transfer from the CORh-binding Rh to the neighboring Au and consequently increased charge donated from CORh to Rh. The varied IR absorption for adsorbed CO can thus serve as an indicator for the charge transfer between the components in bimetallic clusters.

Surface structures and compositions of Au–Rh bimetallic nanoclusters supported on thin-film Al2O3/NiAl(100) probed with CO

The surface structures and compositions of Au-Rh bimetallic nanoclusters on an ordered thin film of Al2O3/NiAl(100) were investigated, primarily with infrared reflection absorption spectra and temperature-programmed desorption of CO as a probe molecule under ultrahigh-vacuum conditions and calculations based on density-functional theory. The bimetallic clusters were formed by sequential deposition of vapors of Au and Rh onto Al2O3/NiAl(100) at 300 K. Alloying in the clusters was active and proceeded toward a specific structure-a fcc phase, (100) orientation, and Rh core-Au shell structure, regardless of the order of metal deposition. For Au clusters incorporating deposited Rh, the Au atoms remained at the cluster surface through position exchange and became less coordinated; for deposition in reverse order, deposited Au simply decorated the surfaces of Rh clusters. Both adsorption energy and infrared absorption intensity were enhanced for CO on Au sites of the bimetallic clusters; both of them are associated with the bonding to Rh and also a decreased coordination number of CO-binding Au. These enhancements can thus serve as a fingerprint for alloying and atomic inter-diffusion in similar bimetallic systems.