# Evidence for “Unusual” Exchange–Correlation on Si(111) 7 × 7: Limitations of Density Functional Calculations for Charge Transfer Interactions on Semiconductor Surfaces

## Abstract

Electron–electron interactions on the Si (111) surface associated with inaccuracies in treating exchange and correlation are shown to be consistent with discrepancies found between density functional calculations and experimental measurements of the 7 × 7 surface. Here, the measured filled and empty surface states of the 7 × 7 surface are shown to be shifted away from one another relative to those predicted by effective one electron calculations using density functional theory. This corresponds to the “energy gap problem” widely known to occur in most bulk semiconductors arising from the approximate treatment of electron exchange–correlation, X–C, within such theoretical constructs. Considering more realistic treatments of X–C to correct these energy shifts leads to a different interpretation of the surface states and a new interaction that stabilizes the 7 × 7. This provides a physical basis for its unusual properties some of which are also observed in related 2D systems. The effects of various electron–electron interactions from these new interactions are considered and their behavior is used to explain the metallic nature of the 7 × 7 at 300 K.