“…The multigrid method (Brandt, 1977(Brandt, , 1982(Brandt, , 1984(Brandt, , 1999Hackbusch, 1985) is a particularly efficient method for solving the discrete equations. Linear-scaling real-space methods have been developed for solution of the Poisson problem in DFT (White et al, 1989;Merrick et al, 1995Merrick et al, , 1996Gygi and Galli, 1995;Briggs et al, 1995;Modine et al, 1997;Goedecker and Ivanov, 1998a). These studies have illustrated the accuracies and efficiencies of the real-space approach.…”