  Despite significant advances in electronic structure methods for the
treatment of excited states, attaining an accurate description of the
photoinduced processes in photoactive biomolecules is proving very difficult.
For the prototypical photosensitive molecules, formaldimine, formaldehyde and a
minimal protonated Schiff base model of the retinal chromophore, we investigate
the performance of various approaches generally considered promising for the
computation of excited potential energy surfaces. We show that quantum Monte
Carlo can accurately estimate the excitation energies of the studied systems if
one constructs carefully the trial wave function, including in most cases the
reoptimization of its determinantal part within quantum Monte Carlo. While
time-dependent density functional theory and quantum Monte Carlo are generally
in reasonable agreement, they yield a qualitatively different description of
the isomerization of the Schiff base model. Finally, we find that the
restricted open shell Kohn-Sham method is at variance with quantum Monte Carlo
in estimating the lowest-singlet excited state potential energy surface for
low-symmetry molecular structures.
