On the electron correlation effects on electronic and vibrational hyperpolarizability of merocyanine dyes

Using various ab initio methods (including many-body perturbation theory and coupled cluster method), the full tensor of electronic and vibrational contributions to the first-order hyperpolarizability $(β)$ for three molecules belonging to the group of merocyanine dyes was computed. Unexpectedly large correlation effects were observed in the electronic counterpart of nonlinear optical response. The analysis of the electronic structure, performed for one of the investigated molecules, reveals that it is the polarity of the low-lying excited state that is affected most upon inclusion of the electron correlation. The vibrational contributions to $β$ are not significantly influenced by the applied level of theory.