  We present a new ab initio method for calculating effective onsite Coulomb
interactions of itinerant and strongly correlated electron systems. The method
is based on constrained local density functional theory formulated in terms of
maximally localized Wannier functions. This scheme can be implemented with any
basis, and thus allows us to perform the constrained calculation with
plane-wave-based electronic-structure codes. We apply the developed method to
the evaluation of the onsite interaction of 3d transition-matal series. The
results are discussed using a heuristic formula for screened Coulomb
interactions.
