DFT schemes based on conventional exchange-correlation (XC) functionals have been employed to determine
the dipole moment (μ), polarizability (α), and first (β) and second (γ) hyperpolarizabilities of push−pull
π-conjugated systems. In addition to the failures already pointed out for α and γ in a recent study on
polyacetylene chains [J.
Chem. Phys.
1998, 109, 10489; Phys. Rev. Lett.
1999, 83, 694], these functionals are
also unsuitable for the evaluation of μ and β. In the case of β, in particular, an almost catastrophic behavior
with respect to increasing chain length is found. We show that the C functional has a negligible effect on the
calculated μ, α, β, and γ whereas the X-part is responsible for the large property overestimations when the
size of the system increases. The overly large μ values are associated with an overestimation of the charge
transfer between the donor and the acceptor whereas for α, β, and γ, incomplete screening of the external
electric field is responsible for the large discrepancies with respect to accurate values. Our results show that
current XC functionals incorrectly describe the polarization of conjugated systems when the polarization is
due to donor/acceptor substitution or an external field or both.
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