Excitation energy exchange-correlation functional

In recent years, the first applications of DFT to excited electronic states of molecules have been reported. In the so-called time-dependent DFT (TDDFT) method, the excitation energies are obtained as the poles of the frequency-dependent polarizability tensor [29], Several applications of TDDFT with standard exchange correlation functionals have shown that this method can provide a qualitatively correct description of the electronic excitation spectrum, although errors of the order of 0.5 eV have to be expected for the vertical excitation energies. TDDFT generally fails for electronic states with pronounced charge transfer character. 

In view of the potential application of TDDFT methods to the spectroscopy of bioinorganic systems, the dependence of the excitation energies on the exchange-correlation functional and the basis set was carefully tested. In Table 16, the TDDFT results are compared to the known experimental data for the Q and B bands of CO-ligated mioglobin (Mb) [163], the SAC-CI calcula-... 

Gonze, X. and Scheffler, M. (1999). Exchange and correlation kernels at the resonance frequency implications for excitation energies in density-functional theory, Phys. Rev. Lett. 82,4416 1419. 

There are a number of model exchange-correlation functionals for the ground-state. How do they perform for ensemble states Recently, several local density functional approximations have been tested [24]. The Gunnarsson-Lundqvist-Wilkins (GLW) [26], the von Barth-Hedin (VBH)[25] and Ceperley-Alder [27] local density approximations parametrized by Perdew and Zunger [28] and Vosko, Wilk and Nusair (VWN) [29] are applied to calculate the first excitation energies of atoms. 

In the widely used adiabatic approximation, which we have adopted here, the time-dependence of the exchange-correlation energy is contained in the density— that is, the exchange-correlation functional is approximated using the same functional form in the time-dependent and time-independent cases. It is not obvious that this approximation holds for other than slowly varying external fields, but it has been verified that the adiabatic approximation is adequate for calculating excitation energies [10]. 

The exchange-correlation functional is inherently local, depending only on the density and possibly its derivatives at a given point, and this causes DFT methods to be inherently unsuitable for describing charge transfer systems, where an electron is transferred over a large distance. Such systems are predicted to have excitation energies that are too low by several eVP... 

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