The role of exchange and correlation in time-dependent density-functional theory for photoionization

Atomic photoionization cross sections are calculated by time-dependent dens ity-functional (TDDF) methods using different exchange-correlation potentia ls including the exact one. The exchange-correlation kernel is treated in t he adiabatic local density approximation (ALDA). Results for the exact full and the exact exchange-only Kohn-Sham (KS) potential are very similar, the calculated photo cross section agree very well with experimental data. Thu s the exact correlation potential seems to have no influence on photoioniza tion and the ALDA for the exchange-correlation kernel seems to be sufficien t for most features of the cross sections. The TDDF method employing the ex act exchange-only KS potential in combination with the ALDA exchange-correl ation kernel therefore is a promising approach to describe photoionization. Deviations from experiment are observed for the widths and shape of the au toionization resonances and have to be attributed to deficiencies of the AL DA exchange-correlation kernel. The calculation of widths and shapes of aut oionization resonances therefore may serve as a severe test for new approxi mate exchange-correlation density-functionals. The asymptotically exact exc hange-correlation potential of van Leeuwen and Baerends also leads to quite good photo cross section, which, however, shows deficiencies close to the ionization threshold and in the energetic position of the autoionization re sonances. Supplementation of the exact exchange potential with the LDA corr elation potential leads to a worsening of the photo cross section because t he LDA correlation potential is too attractive. (C) 2001 American Institute of Physics.