  We outline a computational workflow to model linear optical spectra of
molecules in complex environments in the presence of nonadiabatic effects. The
approach relies on computing excitation energy and transition dipole
fluctuations along molecular dynamics (MD) trajectories, treating molecular and
environmental degrees of freedom on the same footing. Spectra are then
generated following two distinct strategies: In the recently developed Gaussian
Non-Condon Theory (GNCT), the linear response functions are computed in terms
of independent adiabatic excited states, with non-Condon effects described
through spectral densities of transition dipole fluctuations. For strongly
coupled excited states, we instead parameterize a linear vibronic coupling
(LVC) Hamiltonian directly from spectral densities of energy fluctuations and
diabatic couplings computed along the MD trajectory. The optical spectrum is
then calculated using powerful numerically exact tensor-network approaches.
Both the electronic structure calculations to sample system fluctuations and
the quantum dynamics simulations using tensor-network methods are carried out
on graphics processing units (GPUs), enabling rapid calculations on complex
condensed phase systems. We showcase the strengths of the workflow on a series
of two-mode model systems in the presence of a conical intersection (CI), and
the pyrazine molecule in different solvent environments.
