Calculations of the third-order nonlinear optical responses in push–pull chromophores with a time-dependent density functional theory

The third-order resonant and static nonlinear optical polarizabilities of the donor–donor and donor–acceptor substituted π conjugated molecules are calculated using the third-order response formalism in combination with time-dependent Hartree–Fock (TD-HF) and density functional theory (TD-DFT) methods. Performance of different levels of theory for excited state structure and nonlinear optical responses has been analyzed. Since the exact computations are fairly expensive, and only a few components of the cubic polarizability (corresponding to the Liouville space paths) are important, numerically efficient approximations are suggested.