Piris natural orbital functional study of the dissociation of the radical helium dimer

We have investigated the dissociation behavior of the radical helium dimer He(2) (+) using the Piris natural orbital functional (PNOF). This system is particularly challenging to be described by standard density functionals. The restricted open formulation of the PNOF-2, as well as the PNOF-2 energy plus the extended Koopmans' vertical ionization potential calculations of the neutral helium dimer, have been tested for calculating the ground-state energies of He(2) (+) as a function of the internuclear distance. For comparison, we present the dissociation curve obtained with the diffusion Monte Carlo method. The dissociation energies, equilibrium bond lengths, and rovibrational levels are reported. The obtained potential energy curves indicate that PNOF-2 yields a correct and accurate dissociation behavior for the helium radical dimer.