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

TLDR

The study employed conventional and less conventional DFT exchange‑correlation functionals to compute the polarizability and second hyperpolarizability of polyacetylene chains, noting that the short‑sightedness of the XC potentials limits their sensitivity to end‑induced polarization charges. The calculations reveal systematic overestimation of polarizabilities and second hyperpolarizabilities, exaggerated chain‑length dependence, and incorrect bond‑length alternation effects, with failures attributable to both exchange and correlation components of the XC functionals.

Abstract

DFT schemes based on conventional and less conventional exchange-correlation (XC) functionals have been employed to determine the polarizability and second hyperpolarizability of π-conjugated polyacetylene chains. These functionals fail in one or more of several ways: (i) the correlation correction to α is either much too small or in the wrong direction, leading to an overestimate; (ii) γ is significantly overestimated; (iii) the chain length dependence is excessively large, particularly for γ and for the more alternant system; and (iv) the bond length alternation effects on γ are either underestimated or qualitatively incorrect. The poor results with the asymptotically correct van Leeuwen–Baerends XC potential show that the overestimations are not related to the asymptotic behavior of the potential. These failures are described in terms of the separate effects of the exchange and the correlation parts of the XC functionals. They are related to the short-sightedness of the XC potentials which are relatively insensitive to the polarization charge induced by the external electric field at the chain ends.