  We investigate the orientation dependence of molecular high-order harmonic
generation (HHG) both numerically and analytically. We show that the molecular
recollision electronic wave packets (REWPs) in the HHG are closely related to
the ionization potential as well as the particular orbital from which it
ionized. As a result, the spectral amplitude of the molecular REWP can be
significantly different from its reference atom (i.e., with the same ionization
potential as the molecule under study) in some energy regions due to the
interference between the atomic cores of the molecules. This finding is
important for molecular orbital tomography using HHG[Nature \textbf{432},
867(2004)].
