Accurate Prediction of the S1 Excitation Energy in Solvated Azobenzene Derivatives via Embedded Orbital-Tuned Bethe-Salpeter Calculations

By employing the Bethe-Salpeter formalism with a non-equilibrium embedding scheme, we demonstrate that the paradigmatic case of S₁ band separation between cis and trans in azobenzene derivatives can be computed with excellent accuracy compared to experimental optical spectra. Besides embedding, we show that the choice of the Kohn-Sham exchange correlation functional for DFT is critical, despite the iterative convergence of GW quasiparticle energies. We address this by using a global hybrid functional, PBEh, with the amount of exact exchange fulfilling the Koopman’s theorem for DFT hence yielding an environment-consistent ionization potential.
This method yields the first vertical excitation energy of 20 azo molecules with a mean absolute error as low as 0.06 eV, up to three times smaller compared to standard functionals such as M06-2X and PBE0, and five times smaller compared to recent TDDFT results.