First-principles predictions of linear optical spectra of molecules in the condensed phase remains a challenging problem in computational chemistry. The linear absorption and fluorescence lineshapes of chromophores often have vibronic contributions, making an explicit quantum mechanical treatment of the nuclei necessary. Additionally, the condensed phase environment can strongly influence optical spectra, both through environmental polarization effects and direct chromophoreenvironment interactions, such as hydrogen bonding in protic solvents. Slow, generally anharmonic, collective chromophore-environment motion also poses a challenge and is often of importance in complex biological systems. Robust computational approaches capable of capturing these effects from first principles are desirable, as experimental condensed phase optical spectra are often highly congested, making it challenging to establish structure-property relationships from experimental data alone. Additionally, accurately modeling linear spectroscopy often forms the first step in trying to interpret more complicated non-linear experiments used to probe excited state relaxation dynamics in complex systems. A commonly used framework to model optical spectra of molecules is the Franck-Condon (FC) approach. It relies on approximating the ground-and excited state potential energy surfaces (PESs) as harmonic around their respective minima. Nuclear wavefunctions are then a) Electronic mail: tim.zuehlsdorff@oregonstate.edu harmonic oscillator wavefunctions of vibrational modes and the intensity of vibronic peaks is directly related to ground-and excited state wavefunction overlaps following Fermi's golden rule. The methodology accounts for changes in curvature between the ground-and excited state PES, as well as Duschinsky mode-mixing effects. The underlying harmonic nuclear Hamiltonian (referred to as a Generalized Brownian Oscillator Model or GBOM, see Appendix A 1) can be directly parameterized from ground-and excited state geometry optimizations and frequency calculations implemented in many electronic structure methods. Additionally, the exact optical spectrum for the GBOM Hamiltonian can be computed analytically. The method performs very well in predicting linear spectra for small, rigid molecules in non-polar solvents, where the harmonic approximation is expected to hold. However, the approach can struggle in semi-flexible molecules undergoing anharmonic motion, and cannot account for strong solute-solvent interactions, as the condensed phase environment is generally represented collectively through polarizable continuum models (PCMs). To compare directly to experiments, solvent broadening effects have to be accounted for through approximate broadening parameters, or by invoking timescale separation arguments. These shortcomings limit the applicability of the FC approach in large semi-flexible molecules and systems where the chromophore and the condensed phase environment undergo slow, coupled motion.