  Quantum Monte Carlo (QMC) is a stochastic method which has been particularly
successful for ground-state electronic structure calculations but mostly
unexplored for the computation of excited-state energies. Here, we show that,
within a Jastrow-free QMC protocol relying on a deterministic and systematic
construction of nodal surfaces using selected configuration interaction (sCI)
expansions, one is able to obtain accurate excitation energies at the
fixed-node diffusion Monte Carlo (FN-DMC) level. This evidences that the
fixed-node errors in the ground and excited states obtained with sCI wave
functions cancel out to a large extent. Our procedure is tested on two small
organic molecules (water and formaldehyde) for which we report all-electron
FN-DMC calculations. For both the singlet and triplet manifolds, accurate
vertical excitation energies are obtained with relatively compact
multideterminant expansions built with small (typically double-$\zeta$) basis
sets.
