The calculation of the ultra-violet/visible (UV/vis) spectrum of a molecular complex with more than 500 atoms by an ab initio electronic structure methodology is a challenging task. Nonetheless, the correct analysis of the experimental band assignments and the clarification of structure-related properties is crucial. The recently proposed simplified Tamm–Dancoff-Approach (sTDA) to TD-DFT is a highly efficient alternative for the calculation of excitation energies and spectra. It gives the possibility to compute hundreds to thousands of states in a very short post-SCF time span. This approach substantially simplifies the handling of molecular two-electron integrals and truncates the single excitation expansion space. The computational costs are diminished by at least two orders of magnitude compared to standard TD-DFT, while a high level of accuracy is preserved. The sTDA approach was successfully combined with RSH functionals.<br/><br/>The present study reports on the computation of the UV/vis spectra of three merocyanines dyes in solution and in the solid state (thin film) as approximated by large cluster models. The sTDA approach is used for the calculation of the spectra which allows to treat an unprecedented number of atoms (up to 750) in the cut-outs of the crystal lattice. The calculations are done by two methods: first, sTDA employing the ωB97X RSH functional (sTDA-DFT) and second, based on the extended tight-binding Hamiltonian xTB for the ground state calculation (sTDA-xTB). A very good agreement with the experimental UV/vis spectra is reported. The good accuracy of sTDA-xTB is notable since it takes only minutes of computation time for systems with a few hundred atoms and hence is a promising tool in the area of computational analysis of electronic spectra for large molecular structures.

Computational studies of the UV/visible absorption spectra of selected merocyanine dyes

253rd National Meeting (2017)

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