Ab initio density-functional studies of 13-atom Cu and Ag clusters

Dil K Limbu,Michael U Madueke,David A Drabold,Parthapratim Biswas,Raymond Atta-Fynn

doi:10.1088/1742-6596/1252/1/012009

Abstract

The putative ground-state structures of 13-atom Cu and Ag clusters have been studied using ab initio molecular-dynamics (AIMD) based on density-functional theory (DFT). An ensemble of low-energy configurations, collected along the AIMD trajectory and optimized to nearest local minimum-energy configurations, were studied. An analysis of the results suggests the existence of low-symmetric bilayer structures as strong candidates for the putative ground-state structure of Cu13 and Ag13 clusters. These bilayer structures are markedly different from a buckled bi-planar (BBP) configuration and energetically favorable, by about 0.4–0.5 eV, than the latter proposed earlier by others. Our study reveals that the structure of the resulting putative global-minimum configuration is essentially independent of the nature of basis functions (i.e., plane waves vs. pseudoatomic orbitals) employed in the calculations, for a given exchange-correlation functional. The structural configurations obtained from plane-wave-based DFT calculations show a noticeably tighter or dense first-shell of Cu and Ag atoms. A comparison of our results with recent full-potential DFT simulations is presented.

Highlights

Transition-metal (TM) clusters have been studied extensively from computational and experimental points of view [1,2,3,4,5] due to their potential applications in catalysis [6], photonics [7] and carbon nanotubes [8]
While computational studies based on classical interatomic potentials indicate that the icosahedral structure is the preferred ground state of 13-atom Cu and Ag clusters [2, 9], ab initio studies based on the density-functional theory (DFT) [10] indicate the presence of a few competing structures as possible ground-state structures [11,12,13,14,15,16,17,18,19], depending upon the types of the basis functions and exchange-correlation (XC) functionals employed in the DFT calculations
The potential energy of the putative global minimum for Ag and Cu clusters is listed in Table 1, with respect to the potential energy of the corresponding icosahedral structure obtained under identical conditions

Summary

Introduction

Transition-metal (TM) clusters have been studied extensively from computational and experimental points of view [1,2,3,4,5] due to their potential applications in catalysis [6], photonics [7] and carbon nanotubes [8]. Of particular interest among ab initio studies on 13atom Ag and Cu clusters are the work by Oviedo and Palmer [11] and that by Chang and Chou [12], using the first-principles density-functional code VASP [20] The former indicated the presence of a few ‘amorphous-like’ low-energy isomers with bilayer structures, whereas the latter concluded the existence of a buckled bi-planar (BBP) structure as a putative ground state of 13-atom Ag and Cu clusters. A number of Gaussian-orbital-based and plane-wave-based DFT studies [22,23,24,25] reported different structures for Cu13 and Ag13 clusters Toward this end, the aim of the current study is to present results from ab initio molecular-dynamics simulations, lasting for a few hundreds of picoseconds and sampling structural configurations from ab initio potential-energy surfaces (PES), using both plane-wave and local-basis DFT calculations

Objectives

Methods

Results

Conclusion