Partitioning of interaction-induced nonlinear optical properties of molecular complexes. I. Hydrogen-bonded systems.

Understanding the effects of different fundamental intermolecular interactions on nonlinear optical properties is crucial for proposing efficient strategies to obtain new materials with tailored properties. In this study, we computed the electronic and vibrational (hyper)polarizabilities of ten hydrogen-bonded molecular complexes employing the MP2, CCSD and CCSD(T) methods combined with the aug-cc-pVTZ basis set. The vibrational contributions to hyperpolarizabilities included nuclear-relaxation anharmonic corrections. The effect of intermolecular interactions was analyzed in terms of excess properties, which are defined as the difference between a property of the complex and the net properties of the noninteracting subsystems. Considering systems covering a wide range of hydrogen bond strengths, the electronic and vibrational excess (hyper)polarizabilities were decomposed into different interaction energy contributions (electrostatic, exchange, induction and dispersion). This systematic study, the very first of this kind, revealed that the physical origins of the electronic and vibrational excess properties are completely different. In the case of vibrational contributions, the decomposition pattern is very similar for the polarizability and first and second hyperpolarizabilities. The exchange contributions to excess vibrational properties are the largest and they have an opposite sign to the electrostatic, induction and dispersion terms. On the other hand, no general patterns can be established for the electronic excess properties.