MOLECULAR-EXCITATION ENERGIES TO HIGH-LYING BOUND-STATES FROM TIME-DEPENDENT DENSITY-FUNCTIONAL RESPONSE THEORY - CHARACTERIZATION AND CORRECTION OF THE TIME-DEPENDENT LOCAL-DENSITY APPROXIMATION IONIZATION THRESHOLD

This paper presents an evaluation of the performance of time-dependent density-functional response theory (TD-DFRT) for the calculation of h igh-lying bound electronic excitation energies of molecules. TD-DFRT e xcitation energies are reported for a large number of states for each of four molecules: N-2, CO, CH2O, and C2H4. In contrast to the good re sults obtained for low-lying states within the time-dependent local de nsity approximation (TDLDA), there is a marked deterioration of the re sults for high-lying bound states. This is manifested as a collapse of the states above the TDLDA ionization threshold, which is at -epsilon (HOMO)(LDA) (the negative of the highest occupied molecular orbital en ergy in the LDA). The -epsilon(HOMO)(LDA) is much lower than the true ionization potential because the LDA exchange-correlation potential ha s the wrong asymptotic behavior. For this reason, the excitation energ ies were also calculated using the asymptotically correct potential of van Leeuwen and Baerends (LB94) in the self-consistent held step. Thi s was found to correct the collapse of the high-lying states that was observed with the LDA. Nevertheless, further improvement of the functi onal is desirable. For low-lying states the asymptotic behavior of the exchange-correlation potential is not critical and the LDA potential does remarkably well. We propose criteria delineating for which states the TDLDA can be expected to be used without serious impact from the incorrect asymptotic behavior of the LDA potential. (C) 1998 American Institute of Physics.