Cluster study of surface radicals of Si(111)-7 × 7 reconstructed surface

The Si(111)- 7 × 7 surface was theoretically investigated using four different QM/MM surface models. Although low-spin states turned out to be the ground states, there also exist low-lying high-spin states. The energy gap between the low-spin and the high-spin states per dangling bond is nearly constant (∼4 kcal/mol) regardless of the number of dangling bonds in the model clusters, showing that no particular pattern of electron pairing among surface dangling bonds exists. Natural orbital occupation numbers showed that surface radicals due to dangling bonds have ideal open shell configurations. In addition to this, optimized geometries of the low-spin and the high-spin states are nearly identical with each other, suggesting the possibilities of intersystem crossings. So both low spin and high spin electronic configurations are important in the descriptions of the Si(111)- 7 × 7 reconstructed surface. In contrast, geometric differences between pure QM and QM/MM models are significant especially near the adatom region, indicating that the QM/MM embedded model is necessary to adequately represent the Si(111)- 7 × 7 surface especially for small QM region models.