  In this paper we investigate a open two-qubit model whose dynamics is not
exactly solvable. When the initial state is the maximum entangled state, as the
exactly solvable open two-qubit model [D. Tolkunov and V. Privman, Phys. Rev. A
71, 060308(R) (2005)], the decay of entanglement of formation of the model,
expressed by concurrence is also governed by the product of suppression factors
describing decoherence of the subsystems (qubits). However, if the initial
state is not the maximum entangled state, its concurrence will decrease faster
than the product of the suppression factors describing decoherence of the
qubits.
