Response calculations in the framework of time-dependent density-functional theory ͑TDDFT͒ have by now been shown to surpass time-dependent Hartree-Fock ͑TDHF͒ calculations in both accuracy and efficiency. This makes TDDFT an important tool for the calculation of frequency-dependent ͑hyper͒polarizabilities, excitation energies, and related properties of medium-sized and large molecules. Two separate approximations are made in the linear DFT response calculations. The first approximation concerns the exchange-correlation ͑xc͒ potential, which determines the form of the Kohn-Sham orbitals and their one-electron energies, while the second approximation involves the so-called xc kernel f xc , which determines the xc contribution to the frequencydependent screening. By performing calculations on small systems with accurate xc potentials, constructed from ab initio densities, we can test the relative importance of the two approximations for different properties and systems, thus showing what kind of improvement can be expected from future, more refined, approximations to these xc functionals. We find that in most, but not all, cases, improvements to v xc seem more desirable than improvements to f xc .

Efficient real-space approach to time-dependent density functional theory for the dielectric response of nonmetallic crystals Kootstra, F.; de Boeij, P. L.; Snijders, J. G. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Time-dependent density functional theory has been used to calculate the static and frequency-dependent dielectric function ⑀( ) of nonmetallic crystals. We show that a real-space description becomes feasible for crystals by using a combination of a lattice-periodic ͑microscopic͒ scalar potential with a uniform ͑macroscopic͒ electric field as perturbation in a periodic structure calculation. The induced density and microscopic potential can be obtained self-consistently for fixed macroscopic field by using linear response theory in which Coulomb interactions and exchange-correlation effects are included. We use an iterative scheme, in which density and potential are updated in every cycle. The explicit evaluation of Kohn-Sham response kernels is avoided and their singular behavior as function of the frequency is treated analytically. Coulomb integrals are evaluated efficiently using auxiliary fitfunctions and we apply a screening technique for the lattice sums. The dielectric function can then be obtained from the induced current. We obtained ⑀( ) for C, Si, and GaAs within the adiabatic local density approximation in good agreement with experiment. In particular in the low-frequency range no adjustment of the local density approximation ͑LDA͒ band gap seems to be necessary.

In this paper we present a new approach to calculate optical spectra, which for the first time uses a polarization dependent functional within current density functional theory (CDFT), which was proposed by Vignale and Kohn [Phys. Rev. Lett. 77, 2037 (1996)]. This polarization dependent functional includes exchange-correlation (xc) contributions in the effective macroscopic electric field. This functional is used to calculate the optical absorption spectrum of several common semiconductors. We achieved in all cases good agreement with experiment.

Application of time-dependent density-functional theory to the dielectric function of various nonmetallic crystals Kootstra, F.; de Boeij, P.L.; Snijders, J.G.

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