  We study the generation of terahertz radiation from atoms and molecules
driven by an ultrashort fundamental laser and its second harmonic field by
solving time-dependent Schr\"odinger equation (TDSE). The comparisons between
one-, two-, and three- dimensional TDSE numerical simulations show that initial
ionized wave-packet and its subsequent acceleration in the laser field and
rescattering with long-range Coulomb potential play key roles. We also present
the dependence of the optimum phase delay and yield of terahertz radiation on
the laser intensity, wavelength, duration, and the ratio of two-color laser
components. Terahertz wave generation from model hydrogen molecules are further
investigated by comparing with high harmonic emission. It is found that the
terahertz yield is following the alignment dependence of ionization rate, while
the optimal two-color phase delays varies by a small amount when the alignment
angle changes from 0 to 90 degrees, which reflects alignment dependence of
attosecond electron dynamics. Finally we show that terahertz emission might be
used to clarify the origin of interference in high harmonic generation from
aligned molecules by coincidently measuring the angle-resolved THz yields.
