1. A method for calculating spin polarized quantum transport in a 3-dimensional nanoelectronic device under non-equilibrium conditions at finite bias voltage, comprising the steps of: a) self-consistently obtaining the Hamiltonian H of the device using DFT within the standard local spin density approximation for open device structures; b) constructing a non-equilibrium density matrix of the device using Keldysh non equilibrium Green's functions in spin space; and c) calculating spin-dependent transmission coefficients from the Green's functions. wherein said step b) comprises a transverse momentum sampling of the Brillouin zone for contribution of each transverse Bloch state in directions perpendicular to the current flow for converging the density matrix.