  We report first principles investigation of time-dependent current of
molecular devices under a step-like pulse.Our results show that although the
switch-on time of the molecular device is comparable to the transit time, much
longer time is needed to reach the steady state. In reaching the steady state
the current is dominated by resonant states below Fermi level. The contribution
of each resonant state to the current shows the damped oscillatory behavior
with frequency equal to the bias of the step-like pulse and decay rate
determined by the life time of the corresponding resonant state. We found that
all the resonant states below Fermi level have to be included for accurate
results. This indicates that going beyond wideband limit is essential for a
quantitative analysis of transient dynamics of molecular devices.
