  We present and motivate an efficient way to include orbital dependent
many--body correlations in trial wave function of real--space Quantum Monte
Carlo methods for use in electronic structure calculations. We apply our new
orbital--dependent backflow wave function to calculate ground state energies of
the first row atoms using variational and diffusion Monte Carlo methods. The
systematic overall gain of correlation energy with respect to single
determinant Jastrow-Slater wave functions is competitive with the best single
determinant trial wave functions currently available. The computational cost
per Monte Carlo step is comparable to that of simple backflow calculations.
