  A simple experimentally accessible realization of current rectification by
molecules (molecular films) bridging metal electrodes is described. It is based
on the spatial asymmetry of the molecule and requires only one resonant
conducting molecular level (pi-orbital). The rectification, which is due to
asymmetric coupling of the level to the electrodes by tunnel barriers, is
largely independent of the work function difference between the two electrodes.
Results of extensive numerical studies of the family of suggested molecular
rectifiers HS-(CH2)_m-C6H4-(CH2)_n-SH are presented. The highest rectification
ratio ~500 is achieved at m = 2 and n = 10.
