This study explores the utility of Brueckner orbitals as trial wave functions for diffusion Monte Carlo (DMC) calculations. Comparison is made with Hartree-Fock (HF) and density functional theory (DFT) orbitals allowing for an assessment of how well the three sets of orbitals describe the nodal surfaces. For the neutral test systems, PBEO orbitals or Brueckner orbitals give DMC energies that are appreciably lower than those obtained using Hartree-Fock orbitals. For a CO2- anion test case, a significantly lower DMC energy is obtained when using Brueckner orbitals rather than DFT orbitals as the trial function. (C) 2015 Elsevier B.V. All rights reserved.