Jae-Yel Yi

Structural transitions in aluminum clusters

Abstract The energetics of transitions between icosahedral and bulk-like structures is studied theoretically via the Car—Parrinello (CP) method. Total energy calculations show that the energy differences between icosahedral and cuboctahedral structures for 13-, 19-, and 55-atom clusters are surprisingly small, indicating that the transition between icosahedral and bulk structures may occur very early in Al clusters. Bulk embedded atom potentials are found useful for estimates of atomic relaxations for a given structure, but they do not reproduce well the energy differences between different structures. Ionization potentials and electron affinities were calculated within the CP formalism and found to differ by 0.4–1.0 eV from the experimental results.

Simulated annealing strategies for molecular dynamics

Abstract We present a convenient implementation of constant-thermodynamic-speed simulated annealing for molecular dynamics calculations. Sample results are shown for finding the ground states of 13-Al and 55-Al clusters by classical molecular dynamics and by ab initio quantum molecular dynamics using the Car-Parrinello formalism.

Structural transitions in metal clusters

The methods for first-principles calculation of the structure and dynamics of clusters have now progressed to a point where clusters containing ca. 50 non-transition-metal atoms can be studied. As a paradigm, we studied the energetics of structural transformations in 13- and 55-atom Al clusters, which can assume both perfect icosahedral and cuboctahedral structures. Using the Car–Parrinello (quantum molecular dynamics) formalism, we found Al13 has a unique structure, a slightly distorted icosahedron, but Al55 has several inequivalent but energetically nearly degenerate structures. The degeneracy in Al55 is due to the short range of the effective interatomic interactions in a metallic cluster and should lead to floppiness at finite temperatures. A new accurate procedure for calculating ionization potentials (Ei) and electron affinities (Eea) within the Car–Parrinello formalism was developed and applied to Al clusters. Unfortunately, it appears that at least for some clusters, most notably for Al55, the Ei and Eea are very similar for different structural models of this cluster. A formulation and the first tests of a new multigrid-based method for real space electronic structure calculations are briefly described. This method should make possible calculations similar to the above for clusters containing transition metal and/or first-row atoms in the fairly near future.

Structural transformations, reactions, and electronic properties of fullerenes, onions, and buckytubes

Abstract We describe the results of extensive quantum molecular dynamics calculations of the properties of fullerenes and microtubules. The topics to be discussed include: (i) stability of C 60 isomers and barriers to isomerization; (ii) reactivity of C 60 and C 58 with C 2 and C 3 , and its implications on the formation and growth of fullerenes; and (iii) atomic and electronic structure and doping of semiconducting microtubules. We also discuss the structures, stabilities and atomic transformations of large multishell fullerenes and offer an explanation for the formation of spheroidal “onions” under high fluence electron irradiation conditions. The last results, which involved calculations for up to ∼ 15 000 atoms, were obtained using classical three-body potentials.

Quantum Molecular Dynamics Studies of the Structure and Dynamics of Metal Clusters

Publisher Summary This chapter discusses quantum molecular dynamics (QMD) studies of the structure and dynamics of metal clusters. What is present in the chapter focuses on metal clusters with Al chosen as a paradigm. The structures of the clusters will play an important role in the assembly process and influence the properties of the final material. The chapter discusses the structures of larger Al clusters, looking at the icosahedral to fee phase transition via the first-principles Car-Parrinello (quantum molecular dynamics) approach. The results of such calculations can be used for the development of empirical potentials that would be suitable for classical molecular dynamics simulations of much larger clusters, cluster fusion, and the assembly of cluster materials. The chapter describes Car-Parrinello calculations that are focused on (1) investigating the transition between icosahedral and bulk (fee) cluster structures in Al, as a function of cluster size, and (2) on developing a database of first-principles results for determining an embedded atom potential for clusters.

QUANTUM MOLECULAR DYNAMICS OF CLUSTERS

Recent quantum molecular dynamics studies of Al and carbon clusters are described. For Al, we focused on the 13- and 55-atom clusters, which can assume perfect icosahedral and cubic structures. However, the distortions from these ideal structures are substantial. For the 55-atom cluster, several inequivalent but nearly energetically degenerate structures are found, due to the short range of the screened interatomic interactions. For solid C60, it is found that the soccerball structure is well-preserved in the solid. The intermolecular interactions are so weak that the individual C60 can rotate at relatively low temperatures. At high temperatures vibrations cause large distortions, but the cage structure is still preserved. The C60 isomer containing two pairs of adjacent five-fold rings has a binding energy only 1.6 eV smaller than that of perfect C60, but the transformation between these two structures is hindered by a 5.5 eV barrier. It thus requires high temperatures and long annealing times. High temperatures are also needed for the transformation of the lowest energy C20 isomer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precursor for the formation of C60. These results are consistent with the experimental findings that high temperatures are necessary for the formation of substantial quantities of C60. A formulation and the first applications of a new, real space quantum molecular dynamics method, particularly suitable for cluster calculations, are also described.