  In a recent paper, A. Akbari, N. H. March and A. Rubio [Phys. Rev. A. 76,
032510 (2007)] have investigated the one-electron reduced density-matrix and
the momentum density of several two-electron model atoms, including the Hooke's
atom. The method used by the authors for deriving an integral form of the
momentum density is well suited for deriving a closed-form expression of the
exact reciprocal form factor, which function is of importance inasmuch as it
reflects the off-diagonal side of the exact reduced density-matrix.
