  We show that the energy splitting between the bonding and antibonding
molecular states of holes in vertically stacked quantum dots can be tuned using
longitudinal magnetic fields. With increasing field, the energy splitting first
decreases down to zero and then to negative values, which implies a
bonding-to-antibonding ground state transition. This effect is a consequence of
the enhancement of the valence band spin-orbit interaction induced by the
magnetic field, and it provides a flexible mechanism to switch the molecular
ground state from bonding to antibonding.
