Calculations of optical properties of organic molecules and crystals avoidng some drawbacks of TD-DFT and dipole approximations*

Commonly used methods to compute optical properties based on DFT (density functional theory) - time dependent DFT, often used for molecules and clusters, and the dipole approximation, often used for solids - suffer from significant errors having to do with reliance on orbital energies and shapes, as they critically depend on integrals over overlapping orbitals. One consequence of that is strong underestimation of excitation energies with GGA functionals, another is artificial redshift of computed spectra of large molecules or molecular aggregates. We use an alternative approach in which we compute frequency dependent polarizability which does not as strongly depend on orbitals, and from there, the real, then the imaginary part of the dielectric constant, and ultimately the spectrum. We present calculations of absorption spectra of dyes and molecular aggregates that show that this approach is less sensitive to the specific DFT setup than (linear response) TD-DFT and the dipole approximation.