Influence of the Delocalization Error and Applicability of Optimal Functional Tuning in Density Functional Calculations of Nonlinear Optical Properties of Organic Donor–Acceptor Chromophores

Nonempirically tuned hybrid density functionals with range-separated exchange are applied to calculations of the first hyperpolarizability (β//) and charge-transfer (CT) excitations of linear "push-pull" donor-acceptor-substituted organic molecules with extended π-conjugated bridges. An unphysical delocalization with increasing chain length in density functional calculations can be reduced significantly by enforcing an asymptotically correct exchange-correlation potential adjusted to give frontier orbital energies representing ionization potentials. The delocalization error for a number of donor-acceptor systems is quantified by calculations with fractional electron numbers and from orbital localizations. Optimally tuned hybrid variants of the PBE functional incorporating range-separated exchange can produce similar magnitudes for β// as Møller-Plesset second-order perturbation (MP2) correlated calculations. Improvements are also found for CT excitation energies, with results similar to an approximate coupled-cluster model (CC2).