  The electronic structure of two vertically coupled quantum dots containing
two electrons is investigated in the presence of interdot tunneling. Our
approach also includes the interdot exchange and we find that the tunneling is
affected by the Coulomb interaction. There is an interplay between direct
tunneling, the interdot exchange and the Coulomb interaction influence on the
tunneling, which can lead to a zero angular momentum spin-singlet -
spin-triplet transition of the ground state, even in the absence of a magnetic
field.
