  Through the analysis of density of states (DOS), we study two different kinds
of cyclic molecules, a sodium atomic circle and a Polycyclic Aromatic
Hydrocarbons (PAH) molecule respectively under an external magnetic field. The
results of calculation show that original DOS peaks split into two and the
positions of peaks can be modulated periodically by controlling the magnetic
field. We attribute this phenomenon to the interaction between the molecular
orbital magnetic momentum and the magnetic field. Generally, this effect is
obscure for a usual structure but obvious for the cyclic molecules.
