  The eigenstates of an isolated nanostructure may get mixed by the coupling to
external leads. This effect is the stronger, the smaller the level splitting on
the dot and the larger the broadening induced by the coupling to the leads. We
describe how to calculate the nondiagonal density matrix of the nanostructure
efficiently in the cotunneling regime. As an example, we consider a system of
two quantum dots in the Kondo regime, the two spins coupled by an
antiferromagnetic exchange interaction and each dot tunnel coupled to two
leads. Calculating the nonequilibrium density matrix and the corresponding
current, we demonstrate the importance of the off-diagonal terms in the
presence of an applied magnetic field and a finite bias voltage.
