A Study of Applicability of Density Functional Tight Binding—Molecular Mechanics Approach for the Calculation of Optical Properties

Density functional tight binding (DFTB) and time-dependent (TD)-DFTB allow studying optical and electronic properties of materials at scales beyond the reach of Kohn-Sham density functional theory (DFT). Recently, DFTB-MM was introduced in which some atomic pairs are treated via Slater-Koster (SK) parameterizations and some via molecular mechanics, extending applicability of DFTB to cases without accurate parameters for some atomic pairs. Here, we explore the applicability of DFTB-MM to the calculation of optical properties with linear response TD-DFTB. We use a molecule-TiO₂ nanoparticle model system that exhibits an interfacial charge transfer band and modification of optical properties due to molecule-nanoparticle orbital hybridization, as a stringent test for TD-DFTB, which we compare to DFT with different functionals. We find that SK parameters for non-bonded atomic pairs can be omitted with minimal effect on the quality of the spectrum and conclude that TD-DFTB-MM is viable for larger-scale electronic structure and optical properties calculations.