In this talk I will discuss efficient approximations that are possible by applying the tight-binding approximation to time-dependent density functional theory.
Recently we have introduced an oscillator strength based truncation of the single-orbital transition space in order to reduce the computational effort for the calculation of electronic absorption spectra. We show that even a sizeable truncation does not destroy the principal features of the absorption spectrum, while naturally avoiding unnecessary calculation of excitations with a small oscillator strength. The reduced computational cost of intensity selected TD-DFTB can therefore serve to make calculations of optical properties calculations of large molecules possible for a wide range of applications[1].
While the tight-binding approximation is usually made prior to the ground state calculation, this is not strictly necessary. It is also possible to postpone the approximation to the TD-DFT stage and use a method that combines the relatively fast calculation of the electronic ground state in DFT with the efficiency of a tight-binding calculation of the excited states. I will discuss the definition of this approach and its relation to other approximate TD-DFT approaches and show some illustrative results.[2]
References :
[1] R. Rüger, E. van Lenthe, Y. Lu, J. Frenzel, T. Heine, L. Visscher, Efficient Calculation of Electronic Absorption Spectra by Means of Intensity-Selected Time-Dependent Density Functional Tight Binding, J Chem Theory Comput. 11 (2015) 157–167.
[2] R. Rüger et al. (2016), submitted. See arXiv:1603.02571 for a preprint.