With coupled-cluster methods, such as CCSD(T), the calculation of the electron density is not as easy, because the coupled-cluster wavefunction is not variational and the expansion of the exponential operator e T in the expectation value only terminates when all possible excitation levels are exhausted. Thus, a direct calculation of the electron density requires using a linear-response formulation, which is computationally expensive. An alternative is provided by the orbital-optimized coupled-cluster doubles (OO-CCD) approach, in which the single excitation operator T1 is replaced by an orbital rotation operator κ, leading to the introduction of orbital relaxation effects into the theory. The OO-CCD energy is then obtained from the stationary condition respect to both the doubles amplitudes t ij ab and the orbital rotation parameters κ ai , i.e.,