TD-DFT often relies on DFT for ground-state calculations of charge density and structure optimization. Recently, work has been done in tight binding as an approximation to DFT to a) Also at Department of Mechanical Engineering, Massachusetts Institute of Technology, 33 Massachusetts Ave, Cambridge, MA, USA improve its computation time while retaining most of its accuracy. Specifically, density functional tight binding (DFTB) was developed in the late 1990's and exhibited a combination of the accuracy of DFT and the efficiency of semiempirical quantum chemistry methods. More recently, the eXtended Tight Binding (xTB) methods were developed to solve the issues with DFTB of extensive parameterization and low transferability. They differ from DFTB methods in that they utilize top-down parameterization, with semiempirical parameters fit to a large dataset rather than computed with first-principles calculations. The primary approximations are considering molecular orbitals to be a linear combination of atomic orbitals (LCAOs), using the local density approximation (LDA) for exchange-correlation energy, and using a truncated Taylor expansion to map density to total energy. To accelerate excited-state calculations, Grimme introduced the simplified Tamm-Dancoff density functional approach (sTDA) as an approximation to TD-DFT. The key approximations of sTDA include simplifications to twoelectron integrals and setting an upper limit to the excitation space, which improve computation time by 2 orders of magnitude. As sTDA was developed to calculate excitation spectra, there is no excited state relaxation component, and only vertical excitation energies can be calculated. The differences between vertical excitation, vertical emission, and adiabatic energy are shown in Figure . In this work, only vertical excitations are considered. The additional computational expense of excited state relaxation is prohibitively slow for to excited state (S 1 ), with various vibrational levels (0-3) depicted for both states. (b) Demonstration of Frank-Condon principle of 0→1 vertical excitation (blue arrow) followed by nuclear re-configuration and 1←0 vertical emission (red arrow). Also shown is the 0→0 transition energy in yellow. (c) The expected experimental excitation/absorption curve (blue) and emission curve (red), along with the theoretical 0→0 energy difference (dashed yellow line), demonstrating the Stokes shift. Note that in computation, often the energy minimum is used instead of the lowest vibrational level, so the starting energies for excitation and emission may be different in experiment.