  Modeling linear absorption spectra of solvated chromophores is highly
challenging as contributions are present both from coupling of the electronic
states to nuclear vibrations and solute-solvent interactions. In systems where
excited states intersect in the Condon region, significant non-adiabatic
contributions to absorption lineshapes can also be observed. Here, we introduce
a robust approach to model linear absorption spectra accounting for both
environmental and non-adiabatic effects from first principles. This model
parameterizes a linear vibronic coupling (LVC) Hamiltonian directly from energy
gap fluctuations calculated along molecular dynamics (MD) trajectories of the
chromophore in solution, accounting for both anharmonicity in the potential and
direct solute-solvent interactions. The resulting system dynamics described by
the LVC Hamiltonian are solved exactly using the thermalized time-evolving
density operator with orthogonal polynomials algorithm (T-TEDOPA). The approach
is applied to the linear absorption spectrum of methylene blue (MB) in water.
We show that the strong shoulder in the experimental spectrum is caused by
vibrationally driven population transfer between the bright S1 and the dark S2
state. The treatment of the solvent environment is one of many factors which
strongly influences the population transfer and lineshape; accurate modeling
can only be achieved through the use of explicit quantum mechanical solvation.
The efficiency of T-TEDOPA, combined with LVC Hamiltonian parameterizations
from MD, leads to an attractive method for describing a large variety of
systems in complex environments from first principles.
