Benchmarking DFTB+ on the first singlet vertical excitation energy of small organic molecules

Abstract

DFTB+ is a popular and open source scientific software package that offers a fast and scalable implementation of the density functional based tight binding (DFTB) method. Among its multiple implemented features, DFTB+ has a set of tools for simulating the photo-absorption spectroscopy of molecular systems. In this work, we benchmarked these implementations, including extensions to the standard formalism, by computing the first singlet-singlet vertical excitation energy (E1) for the nearly 21,800 small organic molecules in the GDB-8 chemical space. We compared these results with those of less approximate methods from a reference dataset and discussed the nature of the encountered limitations in terms of the approximations made to the parent formalism, density functional theory (DFT). Combining our benchmark results with chemical intuition, we constructed a "rule of thumb" to assist the DFTB+ community of users in selecting the most reliable DFTB-approximate method for the excited state calculations of their molecular systems of interest. For most chemical subfamilies, following this set of recommendations led to E1 estimations with seemingly normal error distributions, mostly contained within ±1 eV . Based on our discussion of the identified limitations, we also proposed to the developer community a possible direction for improvement.