S. K. Lai

Structure and electrical resistivities of liquid binary alloys

Extends the self-consistent energy-independent model pseudopotential theory, developed previously for the binary allows of simple metals, to the calculation of the interionic pair potentials in the K-Rb, Na-K, Na-Cs, Li-Na, Li-Mg, Li-In and Li-Tl liquid alloys for a number of different concentrations. Then the authors determine, for these alloys, the hard-sphere diameters from the calculated interionic pair potentials for use in the determination of the partial structure factors in the Percus-Yevick approximation. Finally, the partial structure factors thus obtained are applied to calculate the electrical resistivities. It is found from the comparison of the calculated electrical resistivities with experiment that (a) the values proposed in this work for the hard-sphere diameters are reasonable and (b) the concentration dependence of the electrical resistivity in the liquid binary alloy, in which the electronegativity difference between two components is significant, is primarily dependent upon the partial localisation of the valence electrons on the electronegative ions in the alloy.

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.

Variational thermodynamic calculation for simple liquid metals and alkali alloys

The authors first propose an approach for treating a correction from the higher-order perturbative terms to the usual low-order pseudopotential perturbation calculation for the one-valence-electron states in a disordered metal. Then, the previously used energy-independent non-local model pseudopotential (EINMP) with this correction is applied along with the original EINMP to the variational thermodynamic calculation in the framework of a hard-sphere reference system for the alkalis in the liquid phase and for the K-Rb, Na-K and Na-Cs liquid alloys. The results obtained for (i) the hard-sphere diameter, sigma , (ii) the isothermal compressibility, chi T, and (iii) the excess entropy, Sexc, are carefully examined and discussed. It appears that in going from the pure alkalis to their binary alloys, (i) the change in sigma follows closely the concept of valence-electron charge transfer, introduced previously to interpret the corresponding concentration dependence of the electrical resistivities and (ii) the deviations of chi T and Sexc from ideality for the Na-K and Na-Cs alloys are largely due to the valence-electron charge transfer, which affects sigma in Sexc and also affects the contribution from the volume dependence of the screening to chi T.

Asynchronous multicanonical basin hopping method and its application to cobalt nanoclusters

The multicanonical basin hopping (MUBH) method, which uses a multicanonical weight in the basin hopping (BH) Monte Carlo method, was found to be very efficient for global optimization of large-scale systems such as Lennard-Jones clusters containing more than 150 atoms. We have implemented an asynchronous parallel version of the MUBH method using the message passing interface (MPI) to take advantage of the full usage of multiprocessors in either a homogeneous or heterogeneous computational environment. Based on the intrinsic properties of the Monte Carlo method, this MPI implementation used the task parallelism to minimize interthread data communication. For a Co nanocluster consisting of N atoms, we have applied the asynchronous multicanonical basin hopping (AMUBH) method (for 181 < N < or = 200), together with BH (for 2 < or = N < 150) and MUBH (for 150 < or = N < or = 180), to search for the molecular configuration of the global energy minimum. AMUBH becomes the only practical computational scheme for locating the energy minimum within realistic computational time for a relatively large cluster.

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.

A self-consistent pseudopotential applied to transport coefficients of liquid binary alloys of alkali metals

The energy-independent model pseudopotential theory, developed and used previously for simple metals, is extended to the binary alloys of these simple metals and a self-consistent pseudopotential, which contains a detailed concentration dependence, is derived for the calculation of various properties of these alloys. This pseudopotential is applied within a low-order perturbation theory to calculate the form factors and transport coefficients for the K-Rb, Na-K and Na-Cs alloys in the liquid state. The calculated results show that a very significant fraction of the valence electrons is localised on the electronegative component in the liquid Na-Cs alloy, as compared with the other alloys considered, and, as a result, the electrical resistivity is very much greater for the liquid Na-Cs alloy than for the liquid Na-K and K-Rb alloys, as demonstrated in experiment. Finally, the applicability of the pseudopotential perturbation theory to the liquid alloys of simple metals is discussed.

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

Use of density functional theory method to calculate structures of neutral carbon clusters Cn (3 ≤ n ≤ 24) and study their variability of structural forms

In this work, we present modifications to the well-known basin hopping (BH) optimization algorithm [D. J. Wales and J. P. Doye, J. Phys. Chem. A 101, 5111 (1997)] by incorporating in it the unique and specific nature of interactions among valence electrons and ions in carbon atoms through calculating the clusters total energy by the density functional tight-binding (DFTB) theory, using it to find the lowest energy structures of carbon clusters and, from these optimized atomic and electronic structures, studying their varied forms of topological transitions, which include a linear chain, a monocyclic to a polycyclic ring, and a fullerene/cage-like geometry. In this modified BH (MBH) algorithm, we define a spatial volume within which the clusters lowest energy structure is to be searched, and introduce in addition a cut-and-splice genetic operator to increase the searching performance of the energy minimum than the original BH technique. The present MBH/DFTB algorithm is, therefore, characteristically distinguishable from the original BH technique commonly applied to nonmetallic and metallic clusters, technically more thorough and natural in describing the intricate couplings between valence electrons and ions in a carbon cluster, and thus theoretically sound in putting these two charged components on an equal footing. The proposed modified minimization algorithm should be more appropriate, accurate, and precise in the description of a carbon cluster. We evaluate the present algorithm, its energy-minimum searching in particular, by its optimization robustness. Specifically, we first check the MBH/DFTB technique for two representative carbon clusters of larger size, i.e., C60 and C72 against the popular cut-and-splice approach [D. M. Deaven and K. M. Ho, Phys. Rev. Lett. 75, 288 (1995)] that normally is combined with the genetic algorithm method for finding the clusters energy minimum, before employing it to investigate carbon clusters in the size range C3-C24 studying their topological transitions. An effort was also made to compare our MBH/DFTB and its re-optimized results carried out by full density functional theory (DFT) calculations with some early DFT-based studies.

Specific heat and Lindemann-like parameter of metallic clusters: Mono- and polyvalent metals

The Brownian-type molecular dynamics simulation is revisited and applied to study the thermal and geometric properties of four mono- and two polyvalent metallic clusters. For the thermal property, we report the specific heat at constant volume CV and study the solid-liquid-like transition by scrutinizing its characteristic. For the geometric property, we calculate the root mean square relative bond-length fluctuation delta as a function of increasing temperature. The thermal change in delta reflects the movement of atoms and hence is a relevant parameter in understanding the phase transition in clusters. The simulated results for the CV of alkali and aluminum clusters whose ground state structures exhibit icosahedral symmetry generally show one phase transition. In contrast, the tetravalent lead is quite often seen to exhibit two phase transitions, a premelting process followed by a progressive melting. In connection with the premelting scenario, it is found here that those (magic number) clusters identified to be of lesser stability (among other stable ones) according to the second energy difference are clusters showing a greater possibility of undergoing premelting process. This energy criterion applies to aluminum clusters nAl=28 and 38. To delve further into the thermal behavior of clusters, we have analyzed also the thermal variation of deltaT and attempted to correlate it with CV(T). It turns out that the premelting (if exist) and melting temperatures of the smaller size clusters (n less, similar 50) extracted from CV do not always agree quantitatively with that deduced from delta.