Reference Energies for Intramolecular Charge-Transfer Excitations

In the aim of completing our previous efforts devoted to local and Rydberg transitions in organic compounds, we provide a series of highly-accurate vertical transition energies for intramolecular charge-transfer transitions occurring in ($\pi$-conjugated) molecular compounds. To this end we apply a composite protocol consisting of linear-response CCSDT excitation energies determined with Dunning's double-$\zeta$ basis set corrected by CC3/CCSDT-3 energies obtained with the corresponding triple-$\zeta$ basis. Further basis set corrections (up to \emph{aug}-cc-pVQZ) are obtained at the CC2 level. We report 30 transitions obtained in 17 compounds. These reference values are then used to benchmark a series of wave function (CIS(D), EOM-MP2, CC2, CCSD, CCSD(T)(a)*, CCSDR(3), CCSDT-3, CC3, ADC(2), ADC(3), and ADC(2.5)), the Green's function-based Bethe-Salpeter equation (BSE) formalism performed on top of the partially self-consistent ev$GW$ scheme considering two different starting points (BSE/ev$GW$@HF and BSE/ev$GW$@PBE0), and TD-DFT combined with several exchange-correlation functionals (B3LYP, PBE0, M06-2X, CAM-B3LYP, LC-$\omega$HPBE, $\omega$B97X, $\omega$B97X-D, and M11). It turns out that CCSD(T)(a)*, CCSDR(3), CCSDT-3, CC3 as well as ADC(2.5) provide rather small average deviations ($\leq 0.10$ eV), CC3 emerging as the only chemically accurate approach. Both CC2 and BSE/ev$GW$@PBE0 also deliver satisfying results given their respective $\mathcal{O}(N^5)$ and $\mathcal{O}(N^4)$ computational scalings. In the TD-DFT context, the best performing functional is $\omega$B97X-D, closely followed by CAM-B3LYP and M06-2X, all providing mean absolute errors around $0.15$ eV relative to the theoretical best estimates.