  We present an approach to the DFT+U method (Density Functional Theory +
Hubbard model) within which the computational effort for calculation of ground
state energies and forces scales linearly with system size. We employ a
formulation of the Hubbard model using nonorthogonal projector functions to
define the localized subspaces, and apply it to a local-orbital DFT method
including in situ orbital optimization. The resulting approach thus combines
linear-scaling and systematic variational convergence. We demonstrate the
scaling of the method by applying it to nickel oxide nano-clusters with sizes
exceeding 7,000 atoms.
