Comment on “Planar tetra-coordinate carbon resulting in enhanced third-order nonlinear optical response of metal-terminated graphene nanoribbons” by G.-L. Chai, C.-S. Lin and W.-D. Cheng, J. Mater. Chem., 2012, 22, 11303

A discussion about the negative sign of the sizable static second hyperpolarizability of a short zigzag graphene ribbon (C60H20) reported in G.-L. Chai, C.-S. Lina and W.-D. Cheng, J. Mater. Chem., 2012, 22, 11303 is presented. As shown, the closed-shell singlet configuration of the specific ribbon as described by approximate methods as the HF and DFT causes serious side effects on the computations of electric response properties such as the dipole polarizabilities and the second hyperpolarizability.