  We introduce a multimodel approach to the simulation of the optical
properties of molecular dyes in solution, whereby the effects of thermal
fluctuations and of dielectric screening on the absorption spectra are
accounted for by explicit and implicit solvation models, respectively. Thermal
effects are treated by averaging the spectra of molecular configurations
generated by an ab initio molecular-dynamics simulation where solvent molecules
are treated explicitly. Dielectric effects are then dealt with implicitly by
computing the spectra upon removal of the solvent molecules and their
replacement with an effective medium, in the spirit of a continuum solvation
model. Our multimodel approach is validated by comparing its predictions with
those of a fully explicit-solvation simulation for cyanidin-3-glucoside
(cyanin) chromophore in water. While multimodel and fully explicit-solvent
spectra may differ considerably for individual configurations along the
trajectory, their time averages are remarkably similar, thus providing a solid
benchmark of the former and allowing us to save considerably on the computer
resources needed to predict accurate absorption spectra. The power of the
proposed methodology is finally demonstrated by the excellent agreement between
its predictions and the absorption spectra of cyanin measured at strong and
intermediate acidity conditions.
