The hyperpolarizability of trans-butadiene: A critical test case for quantum chemical models

The conspicuous fact that the low-order method - the random phase approximation - has served as the best approach to obtain accurate one-photon spectra of the trans-polyenes is scrutinized by performing polarizability and hyperpolarizability calculations of the trans-butadiene molecule using a variety of state-of-the-art correlating methods. The observation persists also for these ground state properties, i.e., the random phase approximation provides results in excellent agreement with experimental data that cannot be matched by higher-order methods. The importance of this observation for simulations of conjugated molecules as non-linear materials is briefly discussed.