  We propose a first-principles time-dependent density functional theoretical
(TDDFT) approach in momentum (P) space for quantitative study of electron
transport in molecular devices under arbitrary biases. In this approach, the
basic equation of motion is a time-dependent integrodifferential equation
obtained by Fourier transform of the time-dependent Kohn-Sham (TDKS) equation
in spatial coordinate (R) space. It is formally exact and includes all the
effects and information of the electron transport in molecular devices. The
electron wavefunction is calculated by solving this equation in a closed finite
P-space volume. This approach is free of self-energy function and memory term
and beyond the wide-band limit (WBL). The feasibility and power of the approach
are demonstrated by the calculation of current through one-dimensional (1D)
systems.
