This paper proposes a general strategy to define molecular orbitals which are especially adapted to the calculation of the energy difference between two states. These orbitals an shown to be eigenvectors of blocks of the difference between the density matrices relative of the two states. They may be used for rational enlargement of the active space in CASSCF calculations or for truncations of the configuration interaction space. Several examples show the relevance of the method to identify the few molecular orbitals of a bridge between magnetic centers which play a role in the spin coupling mechanism.