Absence of bistable behavior in the optical response of a dimer

The problem is examined whether a bistable behavior of optical response of a dimer really exists. We carry out an exact treatment of this problem based on the two-molecule density matrix, without its representation in the form of a product of the one-molecule density matrices of each molecule composing the dimer. Our conclusion is that the bistability discussed is an artifact originating from the splitting procedure. A condition for getting the bistable optical response from a linear chain of a large number of monomers is also formulated.