QUANTUM MONTE-CARLO CALCULATION OF THE FE ATOM

Variational and fixed-node diffusion Monte Carlo calculations for seve ral states of the iron atom are presented. The Ne-core electrons are e liminated by accurate scalar relativistic pseudopotentials. Variationa l valence correlation energies are the largest obtained so far by any variational method including the most extensive quantum chemistry calc ulations. The average difference between experiment and diffusion Mont e Carlo results for the s --> d excitation energies, the ionization po tential, and the electron affinity is 0.15 eV.