Optical Properties of Metal Clusters from First Principles Calculations
Ground State structures of neutral Copper clusters Cu N =3−6 were generated and optimised within the framework of the Density Functional Theory (DFT) using Generalised Gradient Approximation (GGA) and ultrasoft pseudopotential. The shapes and binding energies of the clusters obtained showed good agreement with other theoretical works, except for the trimer cluster which has the shape of an equilateral triangle rather than the isosceles triangle as reported in previous works. The optical absorption strength function of the neutral Copper clusters Cu N =3−6 in vacuum were then computed within the frame work of Time Dependent Density Functional Theory (TDDFPT). The spectrum were computed for the energy range of 1.5eV < ~ω < 5.5eV . The clusters that had odd numbers of atoms showed metallic character and could not be implemented in the turboTDDFT package. Whilst the clusters with even numbers of atoms showed semiconductor property with the Highest Occupied Molecular Orbital-Lowest Unoccupied Molecular Orbital (HOMO-LUMO) gap increasing as the number of atoms change from four to six , and full implementation with turboTDDFT was achieved. In both clusters with even number of atoms four absorption peaks were obtained with that of six atoms higher in intensity. The shift in the direction of the peaks as the cluster size increases shows no unique trend. However, there was a slight broadening of the peaks as the cluster size increased. Most of the peaks were observed to be from the electron transiting from the s-orbital in the occupied state to the orbitals in the unoccupied state. Transitions from low-lying orbitals such as d and p orbital were also noticed.