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We report state-of-the-art quantum Monte Carlo calculations of the singlet $n\to \pi^*$ (CO) vertical excitation energy in the acrolein molecule, extendingthe recent study of Bouab\c{c}a {\it et al.} [J. Chem. Phys. {\bf 130}, 114107(2009)]. We investigate the effect of using a Slater basis set instead of aGaussian basis set, and of using state-average versus state-specificcomplete-active-space (CAS) wave functions, with or without reoptimization ofthe coefficients of the configuration state functions (CSFs) and of theorbitals in variational Monte Carlo (VMC). It is found that, with the Slaterbasis set used here, both state-average and state-specific CAS(6,5) wavefunctions give an accurate excitation energy in diffusion Monte Carlo (DMC),with or without reoptimization of the CSF and orbital coefficients in thepresence of the Jastrow factor. In contrast, the CAS(2,2) wave functionsrequire reoptimization of the CSF and orbital coefficients to give a good DMCexcitation energy. Our best estimates of the vertical excitation energy arebetween 3.86 and 3.89 eV.

Quantum Monte Carlo calculations of electronic excitation energies: the case of the singlet $n \to \pi^*$ (CO) transition in acrolein