The energy distribution of interface states in the Si forbidden gap at ultrathin thermal oxide/Si(111) interfaces is obtained from x-ray photoelectron spectroscopy measurements under bias. All the observed interface state spectra have peaked structure, indicating that they are due to Si dangling bonds. For thermal oxide layers formed at 350 degrees C, only one interface state peak is present near the midgap. The interface state peak has similar to 0.1 eV width, showing that the effective correlation energy of the Si dangling bond interface state is less than similar to 0.1 eV. For oxide layers produced above 550 degrees C, on the other hand, two peaks are observed, one above and the other below the midgap. It is found using a density functional theory method by employing clusters containing 27 bulk-like Si atoms (interior atoms, without H passivation) that an isolated Si dangling bond, with which no atoms in the oxide layer interact, has an energy level near the midgap. It is also found from the calculations that weak interaction of the Si dangling bond with a Si atom having an unpaired electron lowers the Si dangling bond energy below the midgap, while the interaction with an oxygen or Si atom having lone-pair electrons elevates it above the midgap. When the oxide layers are formed at low temperatures, the atomic density of the oxide layer is low, leading to a long distance between a Si dangling bond and the atom in the oxide layer, thus resulting in the isolated Si dangling bond interface state near the midgap. The higher the formation temperature of the oxide layer, the higher the atomic density, resulting in a shorter distance between a Si dangling bond and the interacting atom in the oxide layer. The interface state peaks are shifted from the midgap due to the weak interaction.