  A self-consistent method for calculating electron transport through a
molecular device is proposed. It is based on density functional theory
electronic structure calculations under periodic boundary conditions and
implemented in the framework of the nonequilibrium Green function approach. To
avoid the substantial computational cost in finding the I-V characteristic of
large systems, we also develop an approximate but much more efficient
non-self-consistent method. Here the change in effective potential in the
device region caused by a bias is approximated by the main features of the
voltage drop. As applications, the I-V curves of a carbon chain and an aluminum
chain sandwiched between two aluminum electrodes are calculated -- two systems
in which the voltage drops very differently. By comparing to the
self-consistent results, we show that this non-self-consistent approach works
well and can give quantitatively good results.
