First-principles density-functional calculations for optical spectra of clusters and nanocrystals

Abstract

Electronic and structural calculations for atomic clusters present many challenges for traditional theoretical methods. While the computational framework for ground-state properties of clusters is relatively well established, calculations for excited states remain difficult. In this paper we implement a linear-response theory within the time-dependent local density approximation (TDLDA) and apply this technique to calculate excitation energies and optical absorption spectra for a variety of systems ranging from single atoms to semiconductor quantum dots up to several hundred atoms in size. The TDLDA formalism represents a fully ab initio formalism for excited states that avoids many of the drawbacks associated with empirical or semiempirical methods. Compared to other ab initio techniques for excited states, the TDLDA method requires considerably less computational effort and can be applied to much larger systems. We find the computed excitation energies, photoabsorption spectra, and optical absorption gaps to be in good agreement with available experimental data. Our calculations show that the accuracy of the TDLDA method in the range of lower transition energies is often comparable to that of more computationally intensive techniques, such as methods based on the exact exchange, optimized effective potential, or on solving the Bethe-Salpeter equation within the GW approximation.