Reinvestigation of hydrogen bond effects on the polarizability and hyperpolarizability of urea molecular clusters

The oriented-gas model based on the additivity hypothesis is widely used in predicting the macroscopic nonlinear optical susceptibility of a molecular crystal from molecular (hyper)polarizability calculations. Here, we argue that the intermolecular hydrogen bond interactions will break the additivity relationship for the first hyperpolarizability of urea hydrogen-bonded clusters on the basis of our high-level ab initio studies. The calculations at the HF level without electron correlation correction result in the underestimation of the hydrogen bond effect on the first hyperpolarizabilities to about 15% in the urea case. On the other hand, both HF and MP2 results show the additivity of linear polarizability of urea clusters. We hope that the study will be helpful to the areas of molecular design and simulation of novel nonlinear optical materials.