Assessment of conventional density functional schemes for computing the polarizabilities and hyperpolarizabilities of conjugated oligomers: An ab initio investigation of polyacetylene chains

DFT schemes based on conventional and less conventional exchange-correlatio n (XC) functionals have been employed to determine the polarizability and s econd hyperpolarizability of pi-conjugated polyacetylene chains. These func tionals fail in one or more of several ways: (i) the correlation correction to alpha is either much too small or in the wrong direction, leading to an overestimate; (ii) gamma is significantly overestimated; (iii) the chain l ength dependence is excessively large, particularly for gamma and for the m ore alternant system; and (iv) the bond length alternation effects on gamma are either underestimated or qualitatively incorrect. The poor results wit h the asymptotically correct van Leeuwen-Baerends XC potential show that th e overestimations are not related to the asymptotic behavior of the potenti al. These failures are described in terms of the separate effects of the ex change and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitiv e to the polarization charge induced by the external electric field at the chain ends. (C) 1998 American Institute of Physics. [S0021-9606(98)30447-X] .